Can you elaborate a little on why it would "kill" the motor?
Because if you over carb a motor say 750 manuel secondaries on stock 318 you loose all the vac and kill the motor on the bottom end but if you under carb it you raise the vac and bottom end torque (and up your mpg) and loose the top end. Now this is a little different but same idea with the heads I would think. Kinda like a broke mans stroker lol.

But I am always willing to learn.