I had a 90HP Johnson V4 carbed engine on my last Rib when I bought it. Admitadly a few years youger than your Yamaha however it returned about 36 litres per hour at WOT. (36 knots)
I then had a 115 E-tec fitted to the same Hull and got about the same performance but at higher speeds (40 knots) due to the extra power. To be fair there was not a huge amount in it. Especially at cruising speeds of 25 knots
I would say stick with what you have if its running well.
If you want to know your fuel usage then fit one of these
It has a sender that you fit inline just before the engine and gives an accurate reading of current usage, trip fuel etc.
Handy if you are planning some longer trips.