at 5300 rpms with 1-2ft seas with a half tank, 4 people the boat would do right at 40. Think you could a little more out of it, maybe 45 if there was zero wind and a lighter load.

You should be able to get 6,000 rpms max with the 4 stroke. Is 5,300 the max rpms you could get out of the motor.

no, thats just as hard as he pushed it, he still had some throttle left

If that is the case the you should be fine. I am impressed that the 150 can push a boat of that weight at that speed. I have the same motor (150 Yamaha 4 stroke) on my boat and it has a lot of power/speed.

my dads boat runs wide open at 4400 and 40 mph with a full gas tank