A 350 is a 350 is a 350. What I'm saying is it's a 350 if your at WOT or trolling. Go with the twin 150's. Run as fast as you want and only troll on 1 enegine once you get there. I have twin 150's and don't often see conditions where I can run FLAT OUT, and burn 35 GPH. Twins will make the boat easier to dock and gives you a backup if 1 fails. Even though my engines are the Carbed '97 models, I only burn 1.7 GPH when trolling 1 engine at 3-4 Kts. That 350 is a BRUTE, but I don't feel it belongs on the Gulfstream. You still get 8 Cyls with twin F150's, but you have much more flexibility. I give up the 50Hp just for that
As far as the 200's go ---Yes Yamaha says the MPG/GPH is better, but why feed more then you need to?
BTW --- Grady has told all the dealers NOT to repower any boats built before '08 with the 350. The transom on the earlier boats won't handle the torque.