Not all IHV's impliment the same features in the same ways. There is very little commonality between the fundimental architectures of each IHVs design - especially I'd assume when designing around a fixed console where you can take things like texel cache or the Hiarchial (sp) Z into account, where each IHV has propietory routines, with diffrent levels stored on on-chip buffers. We'll atleast i'd assume so - If I'm wrong correct me please.
Don't forget, we are also dealing with a next generation graphics chip in the XBox 2. So, it probably won't matter as to how it deals with certain routines and such. It's like a Geforce 4 and a Radeon 9700Pro with handling DX8.1 apps. Though different, the Radeon 9700Pro keeps up with the Geforce 4 and in many cases, outpaces the GF4.
But I would also like to say, that's a very good point. Anyone else have more information to add?