Yes, you would have to play it differently. If you get Impact on a target, you have to follow up with one of a specific selection of powers. If I hit a target with Punch and Foot Stomp would be enough to finish it off, but Impact gets applied, now I have to use Haymaker or Jab if I want the AoE damage. If I'm a Brute without a Taunt aura, I have to spread my ST attacks around to grab aggro, but this mechanic requires that you focus your power use on specific targets. This subtly changes how the set feels to play.
Reread the OP - the proposal only adds the mechanic to Jab, Punch, and Haymaker.