Craig:
I wonder how much of the performance difference you are experiencing is the result of the switch up from the 2 to the 4 stroker? 4 strokes are different beasts in my experience. They tend to have better low end "power" but suffer in the top end.
The 6/4 yammy does have a larger engine than the Tohatsu (197 cc vs 123 cc). Might produce more torque? Have you seen a yammy 6 in action?
I know exactly what you are talking about wrt racing for bridges. What you forgot to add is the tide is ALWAYS against you when it's going to be close.