I like the idea of Crits bypassing DR, makes them extra special.
As to powering up casters if DR doesn't affect magic No, but Yes. No it doesn't power up casters, they do the same damage they are doing now to the armor wearer. But yes in the sense that the wizard's melee body guard is now doing less damage to the armor wearer. I think the simplest way would be to just let DR effect any source of damage. Then there is no debate on whether something is magic damage or not. In essence DR/turn would function as an non-magical temp hp source.