Thanks for all the responses.
Also found this artice after a google search.
It's from Yamaha's website, but is a good bit of independant research. Puts the Opti 150 as a good comparison against other 150hp engines on all fronts.
Interestingly shows it as the second most economical of the engines tested (evin 2S/ honda 4S/ yam 2S & 4S), but the noisiest of all(I guess we knew that!)
Anyone else thinking about the same problem as me - have a look at