But the good part of this will be that I will be able to use DX11 vs DX9c, will this make any diffrence?
it depends on the game. there are new features introduce with dx11 (such as tessellation) but not all dx11 games implementing new feature coming from the new API. for the very same quality setting dx11 will have faster performance compared to dx9. so while the game might look just the same in dx9 and dx11 but the performance when running the game in dx11 should be better. here is an example:
Right now, Cryptic has ported over the DX9 engine to DX11. There are no visual improvements or differences in DX11 over DX9 at this time. The porting over however, has the positive effect of boosting performance under DX11. We have tested this to be true with the GeForce GTX 580. You will get a free performance upgrade, simply by switching to DX11 mode. We aren't seeing the reported "20FPS" increase, as the blog states, but we are seeing up to 20% improvements.
i have seen this matter specifically when made my transition but some other that tried to make comparison themselves says some application indeed running slower with win 7. to me it could be caused by the following:
1. old software that are made before windows 7 comes out. they might be able to run with the new OS but since the software was made initially to work with win xp it could be the cause. that's why you can see some older software got patch update so it can work better with the new OS.
2. system with low/weak resource (weak cpu or less RAM). if you're comparing how a certain software between win 7 and win xp with such machine most likely the software will running much faster in XP.