I suspect that you are correct. I wonder where the -20% value came from. My guess is that the action boosts you referred to are a big part of the perceived 'IPS is OP' issue. If so, I like your solution, or just nerf some of the synergy between IPS and the boosts. How about -20% damage for IPS only when EF/NHB is active?
Archer's Focus resetting after each mob is a puzzling choice too, making it effectively a red-named only stance. Why? Having stacks remain when switching targets would not do anything to diminish your stated goal of moving ranged to single-target, as we would still only be engaging a single target at a time. Perhaps a concern of stacking it with Blitz? Then nerf that synergy too. More precise solutions are desperately needed here vs. just crushing all ranged DPS with a big nerf bat.