Well he is right about TM beeing optimized for Nvidia cards, at least in the past. I remember on TMU I could not use max shadow quality with my ATI card because it would show only stipes/lines instead of shadows.
It wasn't really a performance issue, it was simply not showing shadows correctly.
With an older Nvidia card it was showing max shadows beautifully (but was not playable performance-wise).

But, I'm having no problems with TM2 and I play on a C2D @ 3 Ghz and a ATI 4850. Not having everything on max though but still on high quality.