No power

[Cadeco]

The whistle you mentioned when going over 5000 revs is probably an alarm. That is something to look into. My first thought was an over rev condition but that shouldn't occur at 5000 or 5200 revs.
On another note, I did wonder if trim tabs were deployed but figured that was checked.
I agree that you probably are under powered for the load but that doesn't explain the alarm.

That's my thinking

 

[vocz]

Agree, she definitely under power with 7 people and all the weight in the cockpit or stern. You will never get her on plane. Shift people to the bow to get the boat on plane before redistribute the weight.