With the leg you mean I should have heels more down? I'm an eventer at the moment and have done a lot of XC training lately. I'm pretty sure my show jumping form has changed due that one as well :(
All the weigh is in the stirrup ;P It's just that I don't push the heel down but instead "stand" on the ball of my foot. That way it is easier to maintain the two-point and my gravity point stays correct. So in regular jumping I should try to go back with the heel down more to prevent the leg sliding back? It doesn't affect my balance whatsoever in the saddle as far as I can feel.
And it's not just that your heels should be lower, but that all your weight should be resting in your heels. That way if something happens, you might stay in the tack. And while you're right, you're balanced as is, you're balanced on a pivot point and not with a firm foundation under you.
Hope this helps.
ETA: Try clamping your legs against the horse's side to maintain the two point in the field. Try that with the heels down rather than standing in the stirrups.