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.