Honestly, we will probably end up having to agree to disagree. I don't view it as purely physical, when it clearly has had a psychological impact as well. The lesson is simple, if you do X behavior, then punishment is Y, the mind will tie the poor behavior to the consequence and when that behavior starts to appear again, the mind will back the body up and say "hey, let's not get soap in the mouth". Now we have ever only had to do this twice, and they were fairly spaced apart (a year plus if memory serves).
Its great that your method works for your child(ren), and honestly if it worked for mine I would do it. Unfortunately my wife and I got exactly what we asked for, he has my intelligence/curiosity and her personality. The combination of the two makes him a handful because he is smarter than his peers, and considers himself on par with his parents/teachers. This makes trying to get through to him that much tougher because he absorbs things in the moment, but doesn't retain the lesson, so my wife and I are still working to find the right button(s) to push to get the correct behavior.
Now it was brought to my attention last night, as I said earlier my wife is a teacher at the school he goes to, that his class is particularly trying and he is apparently falling in with the kids that are pokers/talkers/poor choice makers. Knowing that, and that his teacher isn't as tough on him as his Pre-K teacher was last year (seriously, that lady did not put up with anything), it changes my view on what is going on and how to approach him to change his behavior. We had a talk last night about how the kids he is sitting next to are part of the problem, and that he needs to make a better choice in who he sits with, when it is his option (they have assigned seats for centers and floor time, but not for lunch or special areas). There is a kid that he gets along with great, and never has a behavior issue from what I can tell, and I asked him why he did sit with him. He replied with "Well Bode doesn't always want to sit with me." To which I asked him to think about it from Bode's view, if Tanner (my son) were getting in to trouble a lot, would Bode want to sit next to him and maybe get in trouble too? I think that was a small "light bulb moment" for him and we will see what happens going forward.