TestFreaks checked out Windows 7 SP1 and concludes there's definitely a difference in performance on their test system. The site found the Service Pack 1 had a negative impact on performance in most of the tests.

"Windows 7 is the best Windows yet, at least that’s my opinion of it. Think what you like, you’re entitled to your own opinion of course. I’m not going to get into an arguing match with you about Win 7 and XP, Vista, OSX etc, this is not what this article is for, take it elsewhere please. If you want to know more about how Windows 7 Service Pack 1 might affect your computer then this is where you want to be. I’ve taken quite a bit of time to run many test on three separate computers with Windows 7 installed, both 32bit and 64bit versions. From my experience I’ve learned a bit and I thought I’d share that with you today, so read on to learn what I learned about how the new Service Pack affects your system."



Link: TestFreaks