For me the CCC almost doubled my framerate. But I assume that depends on the machine used.
For PC's with low ram XP can bring quite some performance boost, as windows 7 uses more ram then XP.
Windows 7 uses less ram if you have less ram in the machine. People use windows 7 on netbooks becuase it gives all the benifits of Win 7, without any effect what so ever on the system. Win7 was in fact made for laptops and netbooks in mind, because they knew how sucky vista was on those type of machines. Microsoft might not be the best corporation, but they wasn't that silly to make an OS even worse than Vista

Not to mention Win 7 comes with x64 which lets your PC/laptop use extra ram if you have it ^^ (Don't talk about the failure of x64 Xp lol)
The only reason not to use Windows 7 is:
1) Your poor/Can't be bothered
2) The program or game you want to play is not supported by Windows 7
I think you will find it's almost always reason 1

And yes as I said, if your machine is really lacking the power to run Crysis/MWLL, then sometimes CCC can let you actually play. But if you can already play it well, there is no point at all, without spending alot of time tweaking settings to make the game look better, not that your notice when blowing that atlas up ^^
Old/Not very good PC = Get CCC. New or ''decent'' PC = Not worth it.