Ok we know a few things:
I figured a 2.57 to 1 reduct gear and you have a 2.51 to 1, close enough.
Your prop is either a 16 by 16 or a 16 by 12.
16 by 12 is much close to my recommended 15 by 10 than 16 by 16.
But while it is clearly still too much prop I'm not sure that it is big enough to cause the problem that you are experiencing. But it is obvious by your symptoms that you are over propped.
Find a reputable prop shop. They can measure and tell you excatly what the dia and pitch are of your existing prop. Ask them if they can resize the prop to 15 by 10. Try that. Or buy a new three blade.
If you really love your boat and you have a birthday coming up, or better yet, your wife has a birthday coming up, giver her a new Max-Prop for her birthday. It's a lot like jewelry. She'll love it. Max-Prop will send you the new prop, already adjusted for what they know to be right for your engine and gearbox. You will love it and you will have far better performance in reverse due to the syrmetrical blade design of the Max-Prop.
And it all ended happily ever after.
Here is what I plan to do; the bottom paint I use works really well, except for the prop and shaft. I assume it is because I don't change the zincs soon enough, the bottom paint wears off quicker on the metal than the hull. So, I have always wanted a second prop so that I could switch them off mid way through the year. Grind and paint the second one and have it put on. So, switch props as they get more difficult to keep clean.
If I buy a second used prop, 15x10, I will have one to switch on and off to clean and repaint. There is a local boat yard that has lots of used props for sale.
Besides, I can tell you right now, the prop as jewelry idea, it ain't going to fly with my wife. So, this is the best I can come up with for now. But, now that you mention it, she wants a Electro Scan waste system, so I can used that and claim it's like jewelery. Works for me.