I've also have this problem. I was making some tests. I had sync issues with AVI but not with MKV, always using HW decoding. Software decoding seems to be synced, but it's a bit rough, like it were losing frames.
Regarding XBMC, I tried and as it uses SW decoding, there's no lag but it's slow too.
I don't know if other fw would work in this box, as there are many c03ref, but I'm not sure if any fw for this board will work without issues.
I've sent an email to the seller but I don't expect any solutions.