The answer seems to be not much when it counts (eg minimum frame rates). Findings seem to show that when you can generate a high enough frame rate to "bottleneck" x8-x8, your framerate is so high the lost frames don't matter.
This article shows a test using a 4.0GHz 920 (to minimize cpu issues) and 2x480s (to provide "ample" gpu power). The mobo allowed choice of "bandwidth", ie, x16-x16, x8-x8, x16-x4. The results are interesting, but no one that I've seen has done the direct 920 vs 750, 2x460 , 1920 resolution comparison.http://www.tomshardware.com/reviews/pcie-geforce-gtx-480-x16-x8-x4,2696-16.html
(The link is mid-article. This is one article where reading the entire thing, and the conclusion of course, may be needed for context.)