I don't know about leg strength, per se, but how about timing? I'm not of the school of thought that I think a fighter has to have huge powerful legs...look at Ray Robinson, Archie Moore...If they keep you from falling down while you do you road work they are strong enough.
But here's the thing, and this is why you feel that loss of power. You'll do fine on the heavy bag because you can predict it's movements. With a live opponent it is much more difficult and this is why there are first rounds and feints and so on. That is how you learn to predict where he'll be and when he'll be there. I'm not trying to be wise here and I hope you see what I am saying.
The other thing is this. It isn't how you push off, it is how you shift your weight to carry the punch. In this case I think "live" legs are better for you than "powerful" legs. You get "live" legs by exercising properly and I think too many of you try to do too much. Don't jump rope on days you spar or run because that is overusing your legs. Don't run and spar. Don't run everyday. Use your legs, each day, and strenuosly, for the number of rounds you are preparing to fight.