John, Yam 115 manual says something like 45 lph at WOT. In practise we used 8 gallons a hour on a heavily laden dive boat, or cruising at high speed
just didn't seem to make much difference.
Now, you might get the fuel figures down a bit by changing to a smaller engine but not a lot without spending alot on a 4 stroke or 2 st injection. Recon Matt is pretty much on the money with that engine, but I would keep it. It is a larger version of the Yam 90 and should run like a sewing machine. They were expensive engines new compared to the other makes and you should now be benefiting from the first owner paying the extra to buy Yamaha. I doubt you will recoup the price of a much newer engine so keep it, polish it, oil it and talk to it and it will bring you home everytime.
There was a fred about fuel figures a while back started by Bruce Hawker, worth a read to see that most folk seem to get between 1 and 1.5 lph.