first boat (pwc) i got was a sts 750. it sure was fun. i went through a jet pump and an engine before i got something else.
check what the other posters said, perhaps check compression too.
with that said. i lost rpms on mine one time, couldn't figure it out. i still remember being able to plane, but she was slow. i gave up and went to seek help at the local repair shop. 1st i went to the local jet ski repair man. not affiliated with any brand and not a dealer. he said water in the fuel. cleaned my fuel tank and charged me. i wasn't to happy, no change in performance. next i went to the dealer. i had a good relationship with the boss mechanic there. in no time he knew what the problem was.
check your flame arrester. the two black plastic snorkels at both ends of the flame arrester. they are oval. the plastic wears and allows them to spin. when they spin the cut off air flow. two ovals that become perpendicular to each other equals reduced air flow. i replaced them and low and behold i got my rpms back. not saying that's the problem, but something simple that drastically affected the performance of my sts 750. good luck.