RE: Widest fixed blade broad head?
Kinetic energy is a little different issue.
Given a 4 blade fixed head (2.5" of cut) vs. a 2 blade fixed head (2.5" of cut) with exactly the same blade angle from the ferrule and exactly the same sharpness of the blade, I believe the math will show that increased diameter of the 2 blade and the increased friction of these blades because they are further away from the center of flight, the 2 blade will require slightly (very slightly) more kinetic energy.
This problem in reality is much more complex. You have expandable heads, different sharpness, different blade angles, different blade thickness, etc. In general, however, it will take more KE for the 2 blade than the 4 blade. And obviously, there is also considerable loss of KEon impactin the case of an expandable to consider. The 4 blade fixed head willlose KE at a more even rateon impact.
I have not done the math, these are just my thoughts based upon my training!