I went from the current 15" Pro that was spec'd out w/ the high-res antiglare screen and 2.3ghz processor upgrade to the 11" i7 w/ 256GB SSD. I also went and bought the Apple Thunderbolt Display to go along w/ this setup.
I am glad I made the switch and I don't feel that I have downgraded much at all. I mainly do database and coding work, so any sort of external monitor is a must. I just happened to get the ATD.
As for performance, I don't notice that much of a hit going to Air from the top of the line 15" Pro. Yea, some compilation and wait times are a little longer, but it isn't too bad. IMO, the performance hit will be noticeable for those who play games or do any sort of video and rendering.
Heat-wise, I would say that the 15" does run about 10 degrees Celsius cooler on the average. Where the 15" would run between 40-50 degrees at idle to low load, the 11" would be at a bigger range of 40-60 degrees, with 50-60 being the average. And this is with the fans running at the minimum of 2000RPM. But in the end, when it comes to games, flash, and video, both machines get equally hot and the fan equally loud.
So why did I do it? Mainly because of the portability and flexibility factor. I found that the 15" was great, and w/ the hi-res screen I did not have to use an external monitor. But it was also heavier and bulkier to carry around in general. As for the 11", this is working out as it performs well enough to be a primary day to day machine that I can either hook up to the ATD or carry around easily without feeling like I have a big laptop.
Conclusion: You can't go wrong either way. It just depends on how you want to work.