No one uses the 'two-leash' method at my club, where one leash is attached to the agitation collar and another to a correction collar. No one uses a harness or wide collar for agitation either. The reasoning there is that the dog associates bitework with those pieces of equipment, and when those pieces of equipment are taken out of the picture, the dog may not perform as well.
A dog will also strongly associate the training field, the tree from which it is tied, the use of a long line, a particular bag being brought out, other dogs being agitated, the clothes you put on, the time of day, the stuff you put in your car, your body posture, your smell (of tension and stress), the sleeve/suit/tug, the agitators movement, the stick, etc. to bitework. It amazes me the level of nuance and subtlety a dog picks up on.
A quick question: How loud do you say "Out" for your dog to let go? How loud have you heard other people say it?
My point is this: make sure you put all the ingredients (equipment) IN the soup; and take MOST of the ingredients out. What you really want is for your dog to associate his aggression to aggression against him or you, or to cues given by YOU. These are the left-over ingredients.
As for the prong...in my opinion, if I need it for capping, it means I don't have the dog I should have. For my dog, I use the prong strictly as a correction, coming from me. As far as helping with other people's dogs, sure, I'll use it for capping. It works best when the dog in question, DOES NOT regularly use a prong collar, is a high prey drive dog, and is also quite hard.