I personally think more focus on recoil, spread, hip fire control, accuracy, handling, ROF, damage per bullet, ADS speed etc should be the way to balance weapons rather than varying HSM's. Hitting headshots should reward higher damage across the board but there's no need for differing multipliers for certain categories of weapons.
If AR's had the same HSM as SMG's what would change? AR's are still better at range and SMG's are still better up close due to the difference in characteristics mentioned above.
IMO a superior HSM on an AR just adds to unnecessary imbalance and its the same with all weapons with varying HSM's.