sal353
I repowered in 2006 my 1999 208 with a 2006 Mercury Verado 150 when I too was concerned about motor weight and enough torque to get on plane. An I4 200 was not available at that time. Here is what I know. I do not have a hardtop on the boat. Viewing the boat without anyone aboard but with equipment and a full tank of gas, the scuppers are not under water. The boat gets up on plane just fine and has a top speed of 38-40 mph with 2 people on board. Gas mileage is 3.8 to 4.0 mpg. Since the Verado is supercharged, it probably generates more torque at low speed to get on plane than the Yamaha F150. If you are concerned about the added weight of the hardtop, you might want to consider the Verado I4 200, which weighs the same as the I4 150. With either of these engines, you get fly-by-wire ignition and nice SmartCraft gauges.
By the way, Grady added about 300 pounds to the front of the boat to accommodate the heavier 4-stroke engines. The dry weight of our boat without hard top weighs 2650 pounds and the later 208s weigh 2950 pounds. Maybe you could add weigh in the bow storage areas if you have a scupper-underwater issue.