I am running into this performance problem also and would appreciate a queryForExtent method on the REST api for Dynamic Map Service Layers. I have some fairly large polygons and even when I use MaxAllowableOffset at a very high number, I still get back a lot more features filled with vertices than if I could just get an extent returned. Any chance of being in 10.3?
Would a geoprocessing service that took a layer and query and returned an extent be faster (I assume it would be less bandwidth) than returning the features to the client and having to post-process them to get an extent?