I have the 50hp etec on my 4.8 ribcraft.
I have quite a large prop on it for distance cruising and economy. Full fuel tanks (thats 3 x 25 litre tanks) and me on board...
30knts, about 1.8nm per litre.
Stacks of oomph out of the hole.
120 hrs and I have used 2 lots of the posh etec oil (68 squids worth so far)
Has not missed a beat.
I like it, but then I would, I bought it.
Some friends have a 75 hp on the back of a Valiant p520
They get 36 knots 2 up. (If they changed the prop it would go bit quicker)
The Valiant is a school boat so it gets a fair amount of hard use. No complaint so far.
I like them so much that I may have to change my Membership to trade status if I go on any longer!