Setting the center channel back further than the mains can not be compensated for in most or all A/V receiver delay settings. This would be the case if the center channel is on the wall behind the TV.
Who's to say they would be further back? I'm not sure what mains or even center he has. I'm sure they make a bracket that will extend out. He can also mount a shelf to the wall and angle the center down.
Definantly get it above the TV and slightly angled downward if you need to. Way to low for a bedroom setting even if you could use your entertainment center. You need to get your speakers up a bit higher in a bedroom setting than in a normal room installation.
Only time I notice it off center is in pro logic and/or reg t.v., which why I switch the receiver to phantom center for that type of viewing. We're still in the remodel stage, soon the center will be above the plasma.