Hahha!! My son wants it to go faster. I must fix it.

Well first thing i would do is a basic tune up....since its fuel injected i would top off with some hi-octane fuel and put some injector cleaner in the tank. Change the plugs/wires and cap/rotor. And finally that air filter looks caked up so take that thing off and clean it good to make sure its getting good air into the motor. If those basic things dont wake it up then i would start doing a compression test on the cylinders to see how far apart they are from each other......im sure its something simple if it runs good now...just need to "unblock" what ever is holding it back :)

You are saying that there is "No great difference in speed between 4000 and 5200"

Are the extra RPMs from 4000 gained by trimming the drive, or also advancing the throttle?

Thank you for answering. The 1000rpm is made from advancing the throttle

How much speed do you gain?

What is the pitch of the propeller?

Did you find out if it is an Alpha or Bravo drive and what the gear ratio is?

I mean , it isn't exactly a racing boat that you've got there but are you sure that nothing much happens in terms of speed over the last 1000 rpm of the throttle range ?
As has been mentioned above , are you using the power trim properly ? You said that it's got an aluminium prop , they're not the best for all round top performance either .
If it was as simple as 'changing a part' I'll make a recommendation assuming that your engine is in good shape and tune .. bolt on a super charger ! A 5.7 engine in that boat isn't much. You really need a big block that can put at least 400hp in the water at the prop to make that thing come alive.