Thank you, we aim to please! I'm sorry I can't help you with this matter.
I have a variation of the same issue myself.
I've got a Dell U2312HM connected via display port to a GTX 660. 1920x1080 is set in both theme and boot.plist.
The Chameleon theme and Apple boot screen show at the correct aspect ratio and there's no stretching, just black space around them.
But here's the weird part, 1920x1080 actually appears on the list of available VESA modes that you can bring up at GUI. Chameleon just doesn't seem to want to use it.
I've determined that it's using 1600x900, which is
not on the list of available modes.