Sorry if I sound pedantic but how large is the performance difference between XP and Vista with TT?
Anyway there seems to be a difference between OpenGL and DirectX when it comes to Aero, the windowing subsystem in Vista.
From what I've heard to get full Aero you'll have to use Microsofts built-in OpenGL driver which only supports version 1.4 and may be slow. If you use any other OpenGL driver, say the speed optimized one that comes with the graphics card, you won't get full Aero but the limitations seem to be minor (tranclucent windows won't work for your program).
This was the situation a while back but maybe there are news.