I had an 88 150 Merc Black Max. It died and I moved up to a 225 2-stroke (couldn't beat the price). I'll have to say that I'd never go back... ever. A 150 on that boat is way underpowered. Sure, it will "push the boat ok," but so will a set of oars. The hull handles the 225 with no problem, as you know since you have one. I have no scupper issues at all. It definately does require trim tabs to cruise in a chop, but in my opinion, every boat needs trim tabs. The minimum I'd put on an overnighter is a 200, and I'll admit that that is probably the optimum power vs weight choce for it. If you ever pull a water skiier, you better give them a snorkel if your doing it with a 150, I cant expect a 175 would be a whole lot better. Seriously, if you are trying to pull a person up on a slalom ski, it is almost impossible with a 150. I'm a pretty good skier, and the time it took for me to get up and out of the water just about drowned me and was just way to difficult (my arms hurt). Also I actually burned more fuel with the 150 during general use. I used to cruise with the 150 wide open almost all the time. Also, it takes a while to get up on plane. With the 225, i can run at 3500 to 4000 to cruise, and usually do, and it pretty much jumps out of the water when you hit the throttle... skiing is much easier. I top out around 47 mph on the GPS. I almost always have a full tank, tons of crap on the boat and usually 4 people on board. Also my hull paint is peeling and in really bad shape (I trailer). If I were to clean up the bottom, I think it will really affect the performance. Not sure if I'll get too much extra speed, but i think fuel burn would decrease a little. I recently installed a flo-scan, so i do have some numbers, however I'm still trying to fine tune it so they may be a little off. I'm getting about 2.31 mpg at 4500 going 33 mph. At 5500 I'm going 44 mph and getting 1.78 mpg. These numbers are with 4 people on board, and a full load of gear.
A little off track there, but my personal opinion is a 200 2 stroke would be the minimum/optimum power I'd put on an Overnighter. The hull handles the weight and HP fine... and I think overall performance is much better with some more power than the 150.
Again, just my opinion.