After playing with a few props I settled on a Yamaha Saltwater Series 2 with SDS. 15 1/2" x 17p. With my configuration I needed more of a bow lift than a stern lift prop. Much better #'s than anything so far. Now turning 5800, 47 mph at 2.1 mpg. This was with 4 persons, 75 gal fuel and running dead into a 15 knot wind and about a 1' chop. With two persons and calm conditions I feel like I could coax 6000 and around 50 mph out of it. 35 to 36 mph yields 2.7 mpg. Optimum cruise was 3800, 30 mph and 3mpg.
Drop to 27 mph and saw 3.1 to 3.2 mpg but boat feels much more responsive at 30+. Hole shot is almost as good as when I was running the twin 150's. Only other thing I may do is to have just a tad more cup put on the tips to see if that allows me to run the motor a little higher. Can't quite run the jack plate high enough to get the ventilation plate level with the top of the water before prop starts to slip.
The SDS is a nice bonus. Although it is not entirely silent gear engagement it is real close. With the Mercs I would cringe every time I threw them in gear. I could just visualize teeth being sheared off the gears. I have owned 7 different Mercs and never got used to that.