Couldn't say in terms of miles per gallon but the fuel consumption on my 115 is scary at higher speeds, cruising at 18-20 knots not too bad, go to 25 and it seems to more or less double, go to 30+kt and it seems to double again!!
The tank on mine is about 80-90 litres I would guess (never emptied it completely but have put 60lt in when showing about 1/4 full on the gauge) and I have used 1/4 tank in an hour and a half pottering round mostly at 10-15 knots then another 1/4 tank in about 20 minutes at "yeeha" setting
If I ever had to replace the engine, or ever got another boat, I think I would go for a four stroke on the basis that it must be more efficient (hard to be any worse!!) and the fact that I don't really like 2 stroke engines anyway