Is there a way to alter the default values...?
The API doesn't go into detail on these values but I would have thought setting the maxImageHeight and maxImageWidth properties on the dynamic layer would cause the layer to request multiple tile to fill the extent but these properties seem to have no effect. If that is the case then something in the JSAPI is broken.
arcgisdynamicmapservicelayer-amd | API Reference | ArcGIS API for JavaScript
Corey,
Are you aware that you are posting in the API for Flex Forum?