The 'less mass' option was a sacrificial anode (for you)

. Surely the difference, if any, will be in modulation and mechanical advantage (dimensions the same remember), rather than power, after all the power comes from your fingers/hand.
I think there may have been a difference between older 105 and Ultegra brakes. Though cables/outers quality/wear level could potentially make more difference than the difference in materials/manufacturing quality. And I suggest that you'd struggle to 'notice' a difference in the current model Ultegra/105, other than a small bit of weight.