Wow, I saw your similar post over on the Mainship Yahoo forum and decided to stay out of it for now and see where it went. But here is a little easier to post and follow the responses, so here goes:
Are you sure that you are talking about the Mainship 400? The 390 came with an integral swim platform that extended the waterline length. Lots of problems with leakage and Mainship probably wishes they never had done it that way.
But I don't think that the factory installed one on the 400T. You might browse Steve Cyr's excellent website dedicated to his 400T at Steven Cyr's Stella Blue Site
. It gives lots of ideas for upgrades but not adding a waterline extending swim platform.
If you were to add one like the 390 has then you might screw up the balance and could certainly affect the rudder response. You would be adding several feet of displacement behind the rudder pivot point, so it can't help but act differently.
And you are also thinking of adding a get home drive there!! Buy a Nordhavn if you want a get home engine.