Are you using countryLayer.setRenderer() to update your symbology?
Also, you should be seeing http 304s and very small response sizes when your feature layer fetches a URL more than once. Are you not seeing 304s?
I am using a UniqueValueRenderer, which I'm not sure is the right thing here (as opposed to a class breaks renderer)
mapLayers.renderer = new esri.renderer.UniqueValueRenderer(defSymbol, "ISO_2DIGIT");
.....
mapLayers.countrySymbols[ccode] = defSymbol;
mapLayers.renderer.addValue({value: ccode, symbol: mapLayers.countrySymbols[ccode],
label: "Label: " + ccode, description: "Description: " + ccode});
.....
countryLayer.setRenderer(mapLayers.renderer);
And then to update I am creating new symbols (as needed, I reuse symbols across the country feature graphics), and assigning them with removeValue() and addValue(), one per country:
mapLayers.renderer.removeValue(ccode);
mapLayers.renderer.addValue({value: ccode, symbol: mapLayers.countrySymbols[ccode],
label: "Label: " + ccode, description: "Description: " + ccode});
But nothing changes onscreen until I do a refresh(), which shows up in Firebug as a full 200 OK weighing in at 310.6K every time in my case. If it's relevant, I have a longish definition expression, to get just the few dozen countries I want.
dojo.io.script.jsonp_dojoIoScript_Countries_generalized_boundaries_10._jsonpCallback({"displayFieldName":"CNTRY_NAME","fieldAliases":{"OBJECTID":"OBJECTID","FIPS_CNTRY":"FIPS_CNTRY","GMI_CNTRY":"GMI_CNTRY","ISO_2DIGIT":"ISO_2DIGIT","ISO_3DIGIT":"ISO_3DIGIT","ISO_NUM":"ISO_NUM","CNTRY_NAME":"CNTRY_NAME","LONG_NAME":"LONG_NAME","ISOSHRTNAM":"ISOSHRTNAM","UNSHRTNAM":"UNSHRTNAM","LOCSHRTNAM":"LOCSHRTNAM","LOCLNGNAM":"LOCLNGNAM","STATUS":"STATUS","POP2007":"POP2007","SQKM":"SQKM","SQMI":"SQMI","LAND_SQKM":"LAND_SQKM","COLORMAP":"COLORMAP","Shape_Length":"Shape_Length","Shape_Area":"Shape_Area"},"geometryType":"esriGeometryPolygon","spatialReference":{"wkid":102100},"fields":[{"name":"OBJECTID","type":"esriFieldTypeOID","alias":"OBJECTID"},{"name":"FIPS_CNTRY","type":"esriFieldTypeString","alias":"FIPS_CNTRY","length":2},{"name":"GMI_CNTRY","type":"esriFieldTypeString","alias":"GMI_CNTRY","length":3},{"name":"ISO_2DIGIT","type":"esriFieldTypeString","alias":"ISO_2DIGIT","length":2},{"name":"ISO_3DIGIT","type":"esriFieldTypeString","alias":"ISO_3DIGIT","length":3},{"name":"ISO_NUM","type":"esriFieldTypeInteger","alias":"ISO_NUM"},{"name":"CNTRY_NAME","type":"esriFieldTypeString","alias":"CNTRY_NAME","length":40},{"name":"LONG_NAME","type":"esriFieldTypeString","alias":"LONG_NAME","length":40},{"name":"ISOSHRTNAM","type":"esriFieldTypeString","alias":"ISOSHRTNAM","length":45},{"name":"UNSHRTNAM","type":"esriFieldTypeString","alias":"UNSHRTNAM","length":55},{"name":"LOCSHRTNAM","type":"esriFieldTypeString","alias":"LOCSHRTNAM","length":43},{"name":"LOCLNGNAM","type":"esriFieldTypeString","alias":"LOCLNGNAM","length":74},{"name":"STATUS","type":"esriFieldTypeString","alias":"STATUS","length":60},{"name":"POP2007","type":"esriFieldTypeDouble","alias":"POP2007"},{"name":"SQKM","type":"esriFieldTypeDouble","alias":"SQKM"},{"name":"SQMI","type":"esriFieldTypeDouble","alias":"SQMI"},{"name":"LAND_SQKM","type":"esriFieldTypeInteger","alias":"LAND_SQKM"},{"name":"COLORMAP","type":"esriFieldTypeInteger","alias":"COLORMAP"},{"name":"Shape_Length","type":"esriFieldTypeDouble","alias":"Shape_Length"},{"name":"Shape_Area","type":"esriFieldTypeDouble","alias":"Shape_Area"}],"features":[{"attributes":{"OBJECTID":45,"FIPS_CNTRY":"HA","GMI_CNTRY":"HTI","ISO_2DIGIT":"HT","ISO_3DIGIT":"HTI","ISO_NUM":332,"CNTRY_NAME":"Haiti","LONG_NAME":"Republic of Haiti","ISOSHRTNAM":"Haiti","UNSHRTNAM":"Haiti","LOCSHRTNAM":"Haiti","LOCLNGNAM":"Republique d'Haiti","STATUS":"UN Member State","POP2007":8706497,"SQKM":27949.040000000001,"SQMI":10791.120000000001,"LAND_SQKM":27560,"COLORMAP":1,"Shape_Length":1588233.8414093608,"Shape_Area":30535891711.168987},"geometry":{"rings":[[[-8138691.4058999997,2228889.4081000015],[-8143052.1421000008,2227346.691300001],[-8166800.5819000006,2228429.5656000003],[-8168223.4750999995,2228560.7611999996],[-8171701.7705000006,2229758.7012999989],[-8174006.0781999994,2231285.5639999993],[-8176108.8989000004,2233223.0943000019],[-8176882.1333000008,2234109.7089000009],[-8178118.7673000004,2236079.8148999996],[-8177746.4166999999,2239890.4375],[-8177515.6500000004,2241421.2894000001],[-8172861.5009000003,2251686.2349000014],[-8171423.4733000007,2253117.0102999993],[-8169707.0177999996,2253330.0311000012],[-8166151.9309,2253461.2608999982],[-8164728.0485999994,2253198.7824000008],[-8162967.1977999993,2253790.4800999984],[-8158421.3593000006,2255565.7047999986],[-8156813.2332000006,2256322.1193999983],[-8147567.7117999997,2261516.6174000017],[-8143546.9486999996,2263818.8687000014],[-8130065.1437999997,2262668.2435000017],[-8124653.0252999999,2262898.5216999985],[-8114851.9170999993,2264015.8933000006],[-8109037.9069999997,2265167.6732000001],[-8106008.0074000005,2266089.1838999987],[-8103873.6676000003,2266252.8614000008],[-8102328.4677000009,2266187.1123000011],[-8091258.1801999994,2263588.5791999996],[-8089990.1391000003,2263161.254999999],[-8079506.9760999996,2259344.7490999997],[-8075116.8482000008,2254743.6314000003],[-8069889.9625000004,2251785.1594000012],[-8050348.2651000004,2244949.2298000008],[-8038010.0671999995,2246313.8137999997],[-8037831.9442999996,2242847.0227999985],[-8036216.8202,2242124.5170000009],[-8025856.9715999998,2240186.0751999989],[-8021466.9930000007,2239989.2802999988],[-8012869.9920000006,2240317.2135000005],[-7994842.3673,2238936.8476000018],[-7987638.9985000007,2238214.4714000002],[-7986957.9513000008,2237426.6327],[-7985674.6451999992,2235883.0824000016],[-7985288.1492999997,2235060.7963000014],[-7985536.0658,2232172.0879999995],[-7985814.3629999999,2228921.6827000007],[-7985503.6696000006,2226919.1374999993],[-7984545.3142000008,2223998.1931999996],[-7984081.7654999997,2222882.2571999989],[-7983587.9666000009,2221766.3865999989],[-7982473.7699999996,2219732.1966999993],[-7982103.4162000008,2218550.9468999989],[-7980711.9300999995,2213695.683699999],[-7980681.5307,2212219.5797999986],[-7981345.4466999993,2202429.8465999998],[-7984236.6175999995,2197366.2901999988],[-7986587.4668000005,2188616.4857000001],[-7983340.0688000005,2185308.1530000009],[-7981175.3296000008,2183408.7291000001],[-7980248.3813000005,2182688.3081],[-7978515.8024000004,2181901.7043000013],[-7977186.0855,2181541.5086000003],[-7973724.0628999993,2180821.3150999993],[-7973738.1896000002,2178938.3376000002],[-7974326.1910999995,2173946.3467999995],[-7974914.1740000006,2171065.6356000006],[-7976536.4453999996,2169003.6919],[-7987173.4528999999,2157257.5456999987],[-7990235.7300000004,2149753.1471999995],[-7983471.6502,2140608.7206999995],[-7982566.8343000002,2130259.4646000005],[-7982627.6143999994,2129084.7065000013],[-7983370.3189000003,2125491.3227000013],[-7984406.7347999997,2121850.5126999989],[-7985194.9729999993,2120528.8786000013],[-7985690.8992999997,2120267.135400001],[-7987854.5000999998,2120136.8068999983],[-7988966.6998999994,2119745.6689000018],[-7989987.8322000001,2119190.1530999988],[-7991596.8354000002,2116644.3407000005],[-7995369.5701000001,2111292.2038999982],[-7996266.2496000007,2110541.4360999987],[-7998677.8789000008,2109855.8957000002],[-8004058.6276999991,2108909.7298000008],[-8005232.466,2109415.8687999994],[-8006578.4370000008,2111104.2533],[-8011726.5342999995,2110736.9235999994],[-8012747.5360000003,2110182.5645999983],[-8015344.4167999998,2107995.6638999991],[-8014663.3695999999,2107017.6506999992],[-8009345.3044000007,2101700.0179000013],[-8003562.6827000007,2095308.2358999997],[-8002372.5715999994,2093473.8691000007],[-8004645.6215000004,2090989.0848000012],[-
etc. etc. etc. - goes on for a long time