if you look on my website ( www.hobbycaptain.at
) you can see my boat (ZAR53) which is comparable from hull length to a normal 6m RIB which has tubes also behind the transom like most UK Ribs.
I owned a Johnson 140 4-stroke (= Suzuki 140) and changed to an E-TEC 150HO.
I am using almost the same fuel with the E-TEC as I used with the Johnson.
Johnson was about 0,4-0,45 l/km , E-TEC is about 0,45-0,5 l/km at normal cruisiung speed between 3000 and 4000 rpm .
Top Speed improved from 75 km/h to 89 km/h (from 40 to 48 kn) and with E-TEC in average at the same rpm I am always 10 km/h faster than with Johnson. This is the major reason for using a bit little more fuel. At the same speed they consume the same amount of fuel.
I never would change back to a 4-stroke any more, I like jumping into plane :-) .
PS: fuel usage is minimum 0,4 l/km with just planing and 2700 rpm (still stratified load) , about 0,5 l/km between 3500 and 4000 rpm (normal cruising speed at about 50 - 60 km/h (27-32 kn) and 0,70 l/km at WOT (89 km/h = 48 kn at 5600 rpm) .
All data recorded with Lowrance 7200C and NMEA2000 engine kit and Navman Powerfuel3100.