There are a few things I've been really zoning in on.
The first is that, like any guy with a weak kickboxing background, I tend to deliver kicks with the top of my foot. There are plenty of professional kickboxers who do that, but it's bad form, especially on the leg kicks.
I was curious where you were taught that instep kicks were bad form? My background is rather traditionalist in my striking, heavily based in Shotokan Karate, and though I will never admit it to him, adaptive, based on my sparring partner's Shaolin Kung Fu.
I ask because I have always been taught that striking with proper form with the smallest most solid area will provide the best effects. In this regard I have for some time been training to land roundhouse kicks with the ball of my feet, I cannot innumerate the number of times I've jammed my toes learning how to do this, but now I am capable of striking with many area of my foot and my shin to in a roundhouse kick.
There is no question that the shin roundhouse is effective, its as you said, like hitting your opponent with a baseball bat. However there is little if any technique involved and thus I have my doubts. In my mind, there is only one reason why the shin roundhouse is powerful, the bone is very nearly right on the surface. In my mind, a shin kick lacks some of the most important part of the roundhouse kick, the rotation of the body and straight-snap motion of the lower leg. According to my experience, using proper form, an instep kick can and is as effective, as well as more surgical than a shin kick. It also allows the user to maintain a safer distance from their opponent(useful in my case since my opponent has 2-3 inches of reach over me), making evasion a much simpler equation. I'm curious as to your opinion on this subject.