Have you seen this article yet? If your interested in some more data and not already aware of the article, they do some pretty good non biased testing on PCIe 3.0 vs 2.0. They Test quite a few games and video card setups.
»www.hardocp.com/article/2012/07/ ··· KE6XC2ms
Like you said, your I-7 920 is most likely bottlenecking you for the games you listed. I only say this because I experience the same thing with my 2500k. When I overclocked it to 4.4 I was able to see an FPS jump of about 10 in BF3. I run skyrim modded out at around 45 fps on a 7950 @ 2560x2440 and I also have 2 other 1080p monitors running in the background. I do have my 7950 overclocked to a modest 1100 mhz. I think the day's of an I5 being all you ever need for gaming are coming to an end, especially with games like Crysis 3, Star citizen, and The Witcher 3 on the horizon.
I have went through much of the same process your doing now for deciding on if I wanted to get the 2011 chipset vs upgrade z77. what I found was for gaming the upgrade costs did not justify the performance gains unless your planning on running 3 to 4 SLI/crossfired cards. I've elected to do nothing until Haswell comes out, but I am toying with adding a second 7950 on my current Z68 motherboard just for fun.
Another thing I have found from reading other people's tests (no testing on my own so it's somewhat opinion that I tend to agree with) is that PCIe 3.0 is approximately 2 times faster then PCIe 2.0. So basically 1 PCIe 2.0 @16x is equivalent to 1 PCIe 3.0 @ 8x. But I have not been able to see any data yet that there is a single card out there that can saturate a PCIe 2.0 @16x lane. From other peoples experiences/testing I have concluded that there does not seem to be a meaningful performance change for z77 vs x79 for gaming until you start running 3 cards in SLI/crossfire. 2 cards in SLI/crossfire seem to have negligable or very small gains in X79 setups vs z77.
If your dead set on upgrading now, my recommendation would be to grab a z77 setup and save yourself some money (unless your going to run 3 or 4 cards). I have elected to wait until Haswell myself due to the very minor upgrade in going from Sandy to Ivy. If you can hold out till Haswell it would probably be best. My bet is haswell will be out before Ivy-E (who knows if they will skip Ivy-E altogether?).
That was my thought process when looking at LGA 2011 chips anyway. I'm curious to here what other people have found too.