It could easily be a very small drain. Harry goes around with that transfigured rock on his finger which is implied to be a not-insignificant constant drain on his magic, yet does not seem to be a noticable detriment to his spellcasting.

Another Test: A persons magic is often likened to a muscle; it gets stronger with use. If the "muscle" could be measured directly would its average strength be higher among Animagi? (Actually testing that would be impossible; there simply aren't enough animagi in the world to get meaningful statistics, but if there were...)

Edit for "Duh" moment. If destroying someone's animagus form is non-invasive, does the former animagus suddenly have slightly more magical power than before their form's destruction?