Mitch: My only modestly educated guess would be that going from 12 to 8 AWG will not make an appreciable difference--though I emphasize that it's only a guess. To answer your other questions, longer runs and thinner wires both increase resistance, and greater resistance means greater likelihood that you'll have a distortive effect on the signal reaching your speakers. Whether that's good or bad is a matter of taste, actually. (A thin cable might roll off highs, and you may like rolled-off highs.)
More resistance will not strain your amp, at least not in the sense that it will push it toward clipping--pushing your amp to clipping would take some doing.
More resistance will not strain your amp, at least not in the sense that it will push it toward clipping--pushing your amp to clipping would take some doing.