ohh and are ther carb or efi engies??

Regardless of how fast either motor gets to 5300/4700, still gotta be going faster than 40mph. Those props don't care what's turning them. If the hubs were bad, I'd think getting on plain would be impossible.
Are you sure the props are indeed 23"? Drive and tabs trimming up ok?

Did it run fine before the rebuild? new/different drives?

The Mirage props are one piece fixed.

he has solid props.....

The engines are carbed. And yes the props are 23. No problem with trimming at all. I just bought the boat and part of the deal was that the engines were to be rebuilt. There was low compression on the starboard engine and so the engines were rebuilt just before I took delivery. I re-checked compression after delivery and compression running at 165 on all cylinders. Fuel or ignition problems might cover the lack of power issues but not the over-revving of rpm's. Right?

Could the tach's be wrong?

Here's a long shot. Could the upper gears on the port side be worn to the point that they slip (or skip) under the high speed load?

Should not be the tach as these are brand new livorsi gauges. At this point I think I will yank the outdrive next weekend and do some further investigation. With good compression and the engine not missing, in my simple mind it has to be south of the flywheel.

your speedo could be wrong or plugged or is this gps reading, 600 rpm difference will slow you down a lot also

Check your timing before you pull the drive. In my mind your problem is in the motor. Check the simple stuff first. We had a guy on the board a few months back that had the throttle linkage hooked up wrong and his issues were almost the same. Take off your air cleaners and with the motors off see if the throttles are opening up all the way. If not look at the other motor and see what is different. After the rebuild did it run ok on the sea trial?

PS on my 311 with 23's that are beat up I can see 70 (speedo not gps) at about 5200 rpm's.