Some of that *kind* of behavior is exaggerated by having them on a leash. Dogs are more aggressive on a leash than off... some kind of trigger in their head, maybe because it is an arbitrary barrier, but I think because it makes them feel constrained while threatened. A dog on a tight lead meeting another dog is far more likely to fight because the line is tight and he feels constrained, than if he had slack. The tight lead makes him think he can't escape either, and is much more defensive. If you let the two dogs meet on loose lines, both their hackles go down. I have seen that a lot of times.
So the punchline of my theory is that he would not be lunging as much if he were loose on a fenceline, invisible or not. You might be right, and he would blow it, but he might surprise you. Electric shock is powerful deterrent, and if he has a fenceline to run, he may run the line rather than bust through.
Make more sense?