'tis strange indeed - I haven't heard of something like this before, but I doubt it's pulseaudio - pulseaudio wouldn't have an incremental effect, just a static freeze rate, so to speak. You could always try killing the pulse process and just have xbmc use your soundcard directly, it's possible (though unlikely) that that was your error. I'm sure a few people who have computers that can even begin to approach 1080p and ran into this problem before will respond soon. I'm no expert in this field, but perhaps you need to enable acceleration on your video card? If the playback previously worked when you had compiz but disabled it, and now doesn't when you chose to go without eye candy from the start, the fact that you never had compiz-like effects enabled in the first place may have something to do with your errror. As far as I know, the only thing choosing "acceleration-enabled" Sabayon does is load compiz at boot and uncomment the acceleration-related lines in your xorg.conf.
As for liteMCE being lite, we meant app-lite; not filled with bloatware, extra DEs, or multiple programs with the same purpose. Compiz can always be disabled, or just installed without.