Illwinter has just ordered a computer with an ATI graphics card. So soon we'll be able to see for ourselves what the deal with ATI and Dom2 is. The crash could be a bug in dom2, SDL, Windows or the ATI drivers, I have no idea really.
Regarding performance my guess is that dom2 uses some OpenGL commands or modes that ATI has not optimized their drivers for and then everything gets real slow. So no matter how much more they optimize for doom3, dom2 will still run slow unless they optimize those slow commands dom2 uses but doom3 doesn't. I'm pretty sure they could make dom2 run faster if they wanted to, but I'm also pretty sure they don't want to.
