You are getting about 3 miles to the gallon in old money. That seems a bit high (I get about between 2 and 2.8 cruising, well thats what the gauges linked to the GPS tell me and down to about 1.6- 1.8 flat out) If you are giving it a lot of stick then that will put consumption up a lot. I have only had 4 strokes but the Avon 5.6 Adventure with F100 Yamaha didn't have the sophisticated electonics to tell me that sort of stuff and i never bothered to work it out - too frightened.
Changing your engine is a very expensive option. You would take forever to recoup the cost in fuel savings. Other things to take into acccount are reliability of a new motor and a warranty, quieter smoother and of course more economical. If it is working ok and does what you want it to I would be tempted to just enjoy it until you need to change it. Those Yam 90 2s seem to go on for ever with a bit of looking after.
I recently had a full re-upholster from Rob Dickinson Sandbanks Upholstery. He is based at Hurn, did a very nice job, quick turn around time and a good guy to work with.
Enjoy your RIB and give us a wave when you are out there.