I have a 1989 GW Sailfish with twin Yamaha 150s. I'm in Virginia Beach so fish and boat on the Chesapeake Bay and close off shore. I know the Gulfstream is 2 feet shorter than my Sailfish but, I can say without hesitation, having twins is paramount. I believe they are similar in height above water so I would expect the Gulfstream to be a "sail boat" in high winds. I'm certain I would not have been able to control the movement of my boat in the wind without twins. There were times, even with twins I was, somewhat, out of control due to the wind.
My parameters when I was looking for a boat were: I wanted two engines but the boat could plane on one. My boat, even with the motors being so old, can, barely, plane on one. I also wanted two separate fuel tanks and two independent batteries. And, "unsinkable"?
I have had single engine boats in the past. I would not want one now. That said, they were both much smaller, less mass, boats so could be maneuvered with one engine fairly well.
But, I would like to re-power my boat! Probably to 175s at most.