I wouldnt like to guess how power scales with the motor size- can be affected by length, diameter, magnet strength, pole count, copper losses etc etc, but I like your theory so lets roll with it
I'd say jump a couple teeth up on the pinion and see how temps do- thats the main decider/ factor that tells you if the motor/esc/batts are happy or being overworked. if temps stay in check then go up a few more teeth and compare temps to see if they've stayed stable or really increased too in line with gearing.