I think the impact of having that big hulkin' dome out on one wing is being somewhat underestimated. There's nothing acting as a counterweight on the other side, so in a way its going to be like flying a P-38 with one engine powered down because the engine is going to be slightly off the center of mass. It may not be significant, but will definitely require more rudder input to keep her straight. Roll rate is almost sure to be affected. And 30mph is a SIGNIFICANT impact on top speed, especially if you consider the calibrated top speed is only off by ~10-15mph.
I don't think the 20mm cannon is going to quite relegate the -5 to the hangar.
And Thor, intentionally crippling one plane's performance to encourage use of another is a very poor approach.