I have the 230hp 5.7 lier Mercruiser and LOVE it. I have been 35 miles off of the NC coast and trolled all day and still had gas in the tank for a short jaunt the following day. When I say troll all day...The motor was started at 0600 and did not get turned off at all until I was back at the marina at 1900!
Drawbacks are the loss of deck space (BIG drawback) and the weight of the motor.
My topspeed is only about 38mph but I rarely put her above a comfortable 25-30 in the ocean.