If you go for a 16' get a Yam 90 on it.
OCR "B Class" run them, best power to weight ratio.
At Ramsgate last weekend they were pushing 60mph.
You'll have to play around with prop's, but that's the same with any set-up.
If you have the time, visit Whitstable 3-4 Sept, OCR are racing, talk to the lads / lassies who have them. There'll be 16's 18's & 19's running for you to compare.