I know it's not limited to sprites. But if the engine can handle 7,000 bobs (or 2,000 with collision detection) fine without pooling, you don't really need to go to the extra length of pooling it. The bobs in my copyPixels() engine only have the properties x,y,frame,HP and objMode, so object instantiation isn't a pressing overhead.

Nevertheless, I'm going to try pooling my copyPixels() engine in a few weeks, once my GCSEs have finished, and see if it makes any significant difference.