One thing is consumption at plane at wot, maintaining plane at 1/2 to 3/4 throtttle, than at just fast displacement boring speeds...
Things to have into account for best fuel usage : sib is properly inflated, well weight ballanced, engine well height seated, trimmed, water conditions.. A 2.70 sib which has more hull drag compared to a larger size, will plane and perform much better with a larger engine than with a 5 HP.
Assume you have tested that combo already ? Does it plane well and fast ? takes its time ? Its a pig on water ?