I am currently ready to buy a 2001 25 outlaw with a 454 mpi
its the 310hp version...I know I Know. my question is can I upgrade this motor to bring it close to 400hp without spending some serious jack or should I pass on the boat

Pass. It would be 56-58mph boat max. You can't upgrade that engine to 400hp for anything less than 3-5k.

Get one with a 454mag, 502mag, or 496mag or HO.

Yes you can upgrade your engine to make 375-400HP.
You will have to change the cam and loose the GM MPI.
The L-29 heads are OK; better than the peanut ports.
Plan on spending all of the 3K Griff suggested because on top of buying a new valve train, carb, intake, and... you should add to the list some better flowing exhaust.
I upgraded a 1999 7.4MPI with a 454HO cam and Stainless Marine exhaust. It gained 200-300rpm on top which was probably 40HP. I probably could have gained another 30 or so if I changed the MPI for a carb.
The GM MPI on the L-29 engine is designed to make low end torque and chokes the engine above 4500rpm.

If you really like the boat buy it...you can probably find a nice, stock low hour 502mpi for around $6k. Subtract from that what you could sell your motor for and you are upgraded pretty reasonably.

+1