I don't know what kind of advantages/disadvantages 4.5.0 has for x264, but I know that Sherpya went back to the 4.2 branch for their mplayer/mencoder builds because of problems getting 4.5.0 to play nice with those or something. I read it on the front page (http://oss.netfarm.it/mplayer-win32.php
), but that was a few months ago. Unfortunately, archive.org doesn't have any revisions of the page from 2009, so I can't double-check exactly when it was.
More than likely you're not going to find substantial differences in compilation of x264 between versions of GCC, any possible miscompilation problems aside. Heck, comparisons I've made between 3.4.5 and 4.4.2 (which were totally unscientific, btw) didn't really show much of a gain in performance - for a couple of tests, 4.4.2 actually was slower
than the 3.4.5 builds by a few hundreths or tenths of a frame-per-second - and this was on a system that's pretty much a dinosaur compared to common setups now. The time spent compiling might be where the bigger difference lies, but I've never done a comparison like that - I mean, it takes my computer the better part of an hour to compile ffmpeg (~5-10 minutes for ./configure, 30-40 for the actual make process), and even though x264 takes maybe 5 minutes tops, I don't really feel like taking up run after run to determine any kind of speed increase in the compiler.
My concern would simply be whether the output from x264 is okay. I can never seem to remember the command to make it run an md5sum check on the output, but that can help you determine if a build you're using is miscompiled (as long as you're using something you know was properly built as your control).