None that I have seen so far. My setup one 24 inch 1080p monitor for gaming and second 17 inch monitor at 720p for temp readings.
Now for ex I benchmarked using heaven and my point score was 925 with both displays. I unplugged the 17 inch and did the same, with 923 points (the 2 points is just a difference what can be ignored in benchmarking)
I had the same question myself and after doing about 7 different benchmarks the results were the same. It doest reduce fps. Another reason is that windows only requires about 64 mb video memory to support 1080p displays and even less for lower resolutions. It is, however good to change your color scheme to classic just for the full juice outta you card. Hope it helps.
+ to badboy... I run dual 24" monitors @ 1080 each web browsing on one while gaming on the other. WoT runs up to 60FPS (and sometimes higher). The only thing I've seen drag it down was running video on the non gaming monitor while in game. That drags the FPS down.
I run my main monitor at 1080 and a secondary mounted above it at 1280x1024. The one above has my steam budy list, maybe a webpage, core temp and afterburner displaying my vitals. I dont see any FPS difference with the secondary screen on or off.