Here one web site that covers the consumtion of various mercruiser models, indicative but gives some ballpark figure.
MerCruiser 3.0 Fuel Consumption 135 HP | MPG GPH US Gallons per Hour | 3.0 L 181 mpi/tks
It would be fun to put a 3.0 L on a SR, the stability would be very good but the weight is maybe litle on the high side?
IMHO the yam 90 is a suprisingly fuel efficient OB for a 2 stroke, and at WOT(33 L/H +- ) there is not much difference even to modern engines.
We use a fuel flow meter on the SR 5.4, and contradictonary what other reports, the speed seams not to affect the economy very much. Sweet spot is 28 knots +-(wheater permitting) and then we do aprox 0,8 l/nm(long distance average, moderate sea satate, fuel and a few kids as cargo). r
At WOT it does aprox 0,85 L/NM and about the same at 22 knots. If less than 22 knots the consumption /nm increases. For sure at cruising speed a 4s or DI 2s would use less.