we've got a Yammy 115 four stroke on the back of a 5.6m Avon adventure (up 4 sale), the engine is ace, extremely quiet and smooth, acceleration is also very good. we use about 20 litres for a good days use, thats with watersports as well. Top speed is around 42 knots 1 up on the absoulte carm, at around 5800 rpm. it will go higher but I won't rev it any further until it has a more suitable prop.
We use about the same amount of fuel, if not slightly less that our 2001 mercury 60hp 2-stroke on 5m Zodiac YL.
I probably wouldn't upgrade to the 115 tho if I was you though, I doubt you will see a considerable increase in top speed that justifys the additional expense. If you do though, opt for a different prop than the standard 13inch, 19 pitch ally prop.