The seal might be good but one Mercury I had was the same, a good seal but the friction of the O ring was slowing the piston down a great deal. The other thing is that sometimes they can be over-sprung, causing piston bounce which reduces power. It's a conundrum to be sure. The game is not helped by the unpleasantness of stripping them and building them, always found them the most difficult to re-spring due to the odd shaped back block and the amount of preload they have.