I'd actually just figured out the problem just last night when debugging other unrelated issues* from a more recent Xorg update. Turns out Debian/wheezy had refrained from installing a firmware blob which was a prerequisite for hardware acceleration. There was a convenient message stating this in Xorg.*.log, which I'd not thought of checking until debugging the new update's unrelated issues.
After installing 'linux-firmware-nonfree', things were suddenly much speedier -- I could no longer see any subjective difference between radeon vs fglrx where video playback/xbmc were concerned. Not clear why linux-firmware-nonfree wouldn't be installed by default given a system configuration that needs it, but I guess that's more of a political issue.
* Something was causing Xorg to fill /var/log with hundreds of Xorg.*.log's. Wiping xorg.conf fixed whatever it was