Hi,
I see in your xorg.conf that you use Xinerama. From what I've read on the Matrox unofficial site, you lose acceleration using Xinerama. It's why I've gone with Mergedfb instead of Xinerama on my Matrox G450 (and I have this way less xruns at low latencies with jackdmp).
You could also use mergefdb
Dual monitors with Matrox