What exactly is the difference between the 2? I know the gimbal ring and related hardware is different, but are the bellhousing, gimbal housing and transom plate the same? Would simply replacing my standard gimbal ring for either Mercs HP or Imcos ring be enough?
I am running an NA 540 with 725 hp and 680 torque. I beat it up pretty well, run the ocean as often as possible. I am getting worried that my standard gimbal will break, or the hinge pins will walk out. BTW, the boat is a '99 259 Convincor.

Do you have twin piston external steering?
If yes, you should be good.
If not, then that is where I'd spend your $$ next.

Yes, I have Imcos dual ram full hydraulic steering.. I should have thought about adding that in the first post.

The dual piston steering adds a lot of stability.
IMHO you do not have to change the transom assembly.

Ttt

The heavy duty gimbal that comes standard on any Mercruiser package from HP500 on up is beefed up in all the right places and is highly recommended. The good steering you have is a big plus.

The HP gimble ring will bolt right up to a standerd transmoe assm. You do lose the place for the elctric trim sender, but most of us have mechanical indictors anyway.