well I had a futura mk 3 for a while with a 25 yam 2 stroke . It was an air floor but it didn't really perform as i had hoped . Probably a much better boat with a solid floor and the smaller MK2 .
The problem with small engines for sibs is that there just isn't any . The current 30 hp 4 strokes are all very heavy, heavier than a tohatsu 50 by quite a bit .
The dealer was going to let me try a new suzuki 25 on the lodestar but he agreed with me that it would be underpowered with any load .
Having the chance of a brand new Tohatsu 50 at a very good price was too good to miss . As for fuel we will have see just how good or bad it is ,
I shall keep an eye out for a 30 mariner , or at least try one if I can as i agree with the small engine idea for touring and carrying less fuel , however I wouldn't want a boat that falls of the plane on the backs of waves , i have experienced that with a loaded sib and its a pain and a little dangerous .
I felt the 25 yam on my Futura was pretty thirsty for the speed it did , probaly because it was working hard all the time , hopefully a bigger motor pushing a hard hull that doesn't reach a wall at about 20kts will do the job .