I wasn't aware/involved in the technical decision. Anyway, what I can tell you is that from a Maya-Softimage-3DSMAX point of view, it's very unlikely that you'll notice a difference. Slightly lowering the video ram speed is only going to affect situations were memory bandwith is the bottleneck (games such as Q3A in high-resolutions, etc..)
In Maya, the bottleneck is more frequently geometry transformation, or driver issues.
Anyway, you can bet that if OEM version are different, it's because those same OEMs have requested it. If they ask to shave off 40$ of each card, by making it slightly slower in some infrequent situations, then ATI will deliver what they want. That's my understanding. (Disclaimer: I'm only a driver guy, I don't have insider knowledge on that issue.)
Christian
OpenGL Driver engineer, workstation products
ATI Research Inc.