Re: repowering - transom build up
Not really a good idea... despite the fact that the boat will take the weight, at least if you adjust your ballast some, that's way too much power.
I'm not saying it'll be uncontrollable, or hard to drive (or not fun) because that's not the issue.
It's a big risk for another reason... a legal one. If you get in any kind of accident at all.... like a drunk boater with four kids steers out in front of you while you have the right of way, you stop completely to let him pass and he turns and rams you.... it'll be your fault.
The courts take a very biased view toward overpowered boats.... putting that much more power on a boat than a manufacturer rates the hull for will get you a very fast hull that could end up costing you everything you own.
Bottom line, if you want to get a faster boat, then get a faster boat... don't try to hop up the one you have.
To answer your actual question, yes you can reinforce the transom to take the 250... but you'll have to rebalance your whole boat so it floats level with the new weight. You'll have less freeboard and take more water over the sides.
Also, the power a boat is rated for isn't related to transom thickness... it's more the hull shape and weight. So even if you make the transom strong and rebalance the hull, you could still end up with an uncontrollable boat, because the person who designed it didn't think it'd ever go that fast, and it's designed to be most smooth and maneuverable at a certain speed.
Think of it this way... your average luxury car has maybe a 300 horsepower engine (on the high end model) and steers very smoothly and easily. If you put a 600 horsepower engine in it, it'll go faster, but do you think it'll be easier to control? Or even feel as stable and smooth as it would at 300?
There's a reason why Ferraris have fancy suspension and controls, and it isn't so they can charge more...
Erik
I am going to add that to your title as well to give it some focus for this section.