I have a small block Merc Mag and an Alpha Drive. I have never owned a Jet Boat but have driven many. To compare is really difficult.
I can say for mileage my Lavey gets about 4 mpg average over a weekend on the Parker Strip verified by my totalizier on my hand held GPS. Two 20 gal tanks will last three days for me and thats driving not constant floating. I can run 70+ but I have a Tunnel Hull boat so small power works OK.