Tks. Subjectively, enabling that makes things noticeably faster.
Here's some informal GTKperf results (default settings), radeonHD (latest from git), ATI X1950XTX, Intel Core2Duo E6600, Ubuntu 7.10:
XAA (XAANoOffscreenPixmmaps) 11.7
I don't know if it's always been like this, but latest driver from git with "AccelMethod" "EXA" added, cripples my computer to the max.