Think of driving a car in say 3rd gear - pretty good acceleration but top speed not so good. Now think of 5th gear - great for top speed but useless if you need to accelerate out of the way of a falling tree!!!
In rough seas you won't be going very fast but you need instant response and the higher the pitch of prop the worse it gets. To be honest I reckon Redbay will already have got the best compromise they can out of those engines.
Trim tabs generate lift at the stern - this will make the hull run flatter and lift the arse end - this MAY overcome the extra drag they create but no guarantees. Still triim tabs are a good thing to fit anyway - if your bow is light it is good to keep the nose down into a head sea.
£453 inc vat - expensive but cheaper than a dual ram Bennet or similar and easy to fit - to the right hull. You will have to check they will fit first - how much people will charge you is anyones guess!!!
I would buy the boat IF the price is right and look into chipping the engines - it should be possible but the marine market is much smaller than the car market so finding one may not be easy.