I have owned the following two MBAs:
- Mid-2011 i7, 4GB, 256GB
- Mid 2013 i7, 8GB, 256GB
Both ran Windows 7 100% of the time and they ran it flawlessly. They perform as expected, i.e. they run as fast as any similarly specced Windows PC would. Windows is not more taxing than OS X, so if you're happy with OS X performance you should be happy with Windows performance.
I might add that I work as an engineer/developer, so I've run a lot of stuff on these machines, not just web browsing and playing music. 🙂
EDIT: And I know you asked about Win 8/10. Win 8 is fully supported by Apple and does not run slower than Win 7. Regarding Win 10 support, I can't really say. I'm guessing there will be official Boot Camp support once it launches. I would expect it to run just as well as Win 7/8.