The Abominable Snowman
Member
Keeping shader units fed has little to do with external bandwidth if you have a 32MB high bandwidth low latency buffer to render into. Because you don't necessarily need to store a large amount of data to keep them fed. Over 320 SP's would be far from a waste IMO.
There's also the question of power usage. There's a curve where increasing the SPs increases the power usage, and thus you'd have to reduce the power envelope of the whole system in order to fit within power constraints. There's also concern for heat and whatnot with all components being under the same xclamp/heat spreader.