It has been discussed here with the before with the 150 Optimax (2.5l V6)
Can a 150 Optimax be upgraded to 200?
super chippin mariner engine
I would imagine for it to be possible to upgrade the 225 you would have to get the ECU from another similarly built engine ie: (3.0l V6)
Maybe an ECU from a Mercury Promax or a 250XS. You will also have to change the exhaust as well as these engines have racing exhausts, not to mind a host of other racing parts to produce the extra power.
Overall it is a very costly procedure and will void your warranty. A new more powerful engine would work out cheaper.