Canlt help out with the 5.4 comments, but to add to the soup:
My old SR4 had a 25. it went "ok", but not particularly fast, and with absolutely no more than 2 aboard.
Many moons ago on a Sportsboat course (shows you how long ago it was!
) they had an SR5.4 with twin 30s. (standardised on Yam 30s on all boats in the centre) Unfortunately they were the last two premix engines on site, and the week before one of them got a drink of neat fuel. Net result was that we used it 4 up with a single 30. It went, it planed, but it was a " bring a good book" moment.......
So if you are getting it for a song, it will probably get you by.