I've been tinkering around with the old firmware on the Xai and I get the impression that accel is gone on 5040 (native?) dpi.

http://www.megaupload.com/?d=UDIGHOM0

Here's the firmware. It's a really old one and the max dpi is 5040 (like Avago spec sheet) instead of 5001.

Would be interesting if you guys could give some more feedback on this.