I used to own a 1988 Offshore. With sprayed on Teflon bottom paint (very smooth) boat would do 44mph as per GPS with 1988 225 Evinrude spinning a 15X17 aluminum prop. This was a notched transom boat, with no hardtop. This was lightly loaded with 50 gallons of fuel in flat water.
I am no expert, but in almost every performance test I haver read, when comparing the same boat with twins to a single outboard the findings are similar. Here are my observations: Boat A. has 225 single, to achieve same top end boat needs about twin 125hp (if available). Boat B. has 250 single, to achieve same top end needs about twin 140hp (if available). This "need" for twins to have more total horsepower is due to added weight and added drag.
Most boat owners (especially Grady owners) do not operate a boat at full throttle all that often. However it is a a good measurement of overall boat performance. I had very few worries about heading offshore with my single. But given the choice on that boat (or my current 24' Explorer) I would prefer a set of new 140 or 150 hp. Not necessarily for the speed or the redndancy, but for handling and choices. You could run on one when slow trolling, docking would be greatly improved, and 'somtimes' fuel economy actually improves.
Tunaman, trailer boats magazine did a comparison on the same boat between a Yamaha F225 and two F115's. The two F115's out performed the single F225 in fuel economy, topspeed and time to plane.Pretty good article sometime last year. Don't remember the boat they used right off hand.
Gulfcoast 242G, My vote (but not a choice) would be for a pair of 140HP four stroke suzuki outboards. Currently best power to weight ratio of the four stroke outboards.
I was a little hasty with my response above. With today's four strokes and hi presssure outboards things are changing. Some of the newer large outboard powered boats are actually more efficient with triples instead of twins. Guess I'll have to remember that when I win the lottery and pick that 36' Grady up...