140 suzuki on 20 MA

[McGillicuddy]

I'm no aerodynamics expert, but consider your large t-top is likely affecting your speed in both drag(acting like a parachute) and exacerbating bow-lift(more like a lifting foil) simultaneously pushing your stern down. That would account for the front end getting squirrely at speed. Likewise trimming up (bow up) would theoretically increase the drag on the top.

What prop model are you using? I would guess with the extra weight of the 4-stroke, albeit a light one, you want a wheel that lifts the stern a bit. Mounting height-wise, 2" might be a little high but with a big wheel it might not hurt too much.

If you can easily remove the top itself you might try that and give it another whirl. I'm guessing you'll easily pick up 3-5 mph.