No bottlenecks will be had. In SLI you'll be at 8x/8x config, and again it won't matter. I'm running a pair of overclocked 5850s without issue on an i5 750. Reviews show the 8x lanes make very, very little difference. Between P55 and X58, with 5870s in the slots, the 8x/8x vs 16x/16x was less than 5% worse which can be attributed to other things like a slower CPU, slow DMI, or other stuff.
Anyway, don't worry about a bottleneck. Just make sure your mobo is SLI certified.
Yes of course. the 1156/P55 platform supports 16 PCIe lanes. With a single card you get all 16 lanes in one slot, with two cards you split the lanes to 8x/8x. 16 lanes provies more bandwidth, but like I was saying that doesn't really matter since most GPUs don't fill up 8 lanes' bandwidth.