According to the Sage, enhancement bonuses for the dastana and the cha-watchamacallit (those armour pieces from OA) overlap with the enhancement bonuses for regular armour. This is reasonable enough IMO. Would it also be reasonable to extend this ruling to shields? At the moment, the stacking rules for armour bonuses make magic shields a bit too good for my tastes, especially at high level -- a piddly buckler, enchanted up to +5, is better than a breastplate (albeit only against one opponent).