I don't suppose Bezel Management is fixed with your technique however, uh ? (broken picture on the left screen past a certain value)
I suspect the Bezel Management issue is a timing problem (running at max as it is) as it doesnt happen in 4320x900.
If you want an elegant fix for the GXM reseting res issue (and not adding 5040), look at
DeviceInfo.ini in your Powerdesk install folder. All the hack Matrox use to gain support on both nVidia and ATI are there in the form of Reg entries... which is also why the 180 series dont work, the reg key changed slightly.
Would take someone at Matrox 2min to correct that, as we have proven the actual 5040 res is functioning as it always did.
Edit: Made up a dirty kludge to fix the resetting issue, see here:
http://www.widescreengamingforum.com/forum/viewtopic.php?t=14961