[@Bartimaeus] He increases SPEED by lowering WEIGHT. As such the FORCE applied stays the same, so does the MASS of his hand. So when you increase gravity again, your FORCE will stay the same, expect this time you'll have more WEIGHT but lose a lot of SPEED to increased gravity. MASS and FORCE stay the same.

If you wanted to dou and uppuercut or strike from above, then gravity would matter, since there would be no horizontal bleed.

Simply lowering friction would make for a stronger punch. Boom, that easy.