For someone who does not use there boat a lot, your return on investment in regard to fuel economy is never and beyond. If you choose to upgrade to the 4s, keep the old EFI on a stand in the garage(pickled)
and put it back on the boat when you do sell. A 4s on a 20 year old boat will not be appreciated as much as it should IMO.
Boat shopping in the future with a 4s to put on whatever you buy will put a lot more boats in play, perhaps a great deal on one with a blown Opti.