WOW! There is a lot going on in this discussion. "The Optimal Slalom Boot" is probably not going to be any one particular boot, but rather a set of characteristics that you can use to seek a boot that works best for you. I think that is what Adam was looking for when he started this discussion. Everybody's input about what they have tried and what changes they have made can help any of us with our search for that perfect boot performance. You also have to consider fit and comfort, as those are also important!
I already mentioned that I have always sought great forward flexibility in the boots I use. That has allowed me the means to get my COM over my front foot. For the most part that has always meant modifying my skate boot shells to allow that. The stiff forward/back support was always too much for my style. I will work on posting photos of modifications on my website.
After many years of using double hard shell boots, I recently switched to a half boot style rear. When I was younger and more flexible, I had no problem with this setup and most younger(40-45 & under) would not have a problem making this setup work. However in more recent years, as my flexibility has diminished, I found that the full rear hardshell boot was causing my leg/shin to influence the roll of the ski and completion of the turn. This lead to my setup change.
@Wish I like what you have done with the rubber strap. I think that could help me with controlling excessive heel rise problems in some instances. Since you were not on that setup when I skied with you recently, I expect you are still testing your performance on that. As everybody can see in my photo, I have gone with a very low cut to my rear boot. This has further helped with the adverse roll caused from a hardshell. I had started that direction after using the Radar HRT, and went even more that way after discussing it with Adam.
@ALPJr several people are using the double Vapor boots on a double release system, and several are on a single Vapor boot on a single release system with r style or rtp. Using a Vapor boot in the rear with a front release system might work for you if you NEVER tighten the upper rear laces or REMOVE them completely. The purpose of using the release system on the front would be so that you can tighten the front boot as much as you want, but the back needs to remain loose. The same is true for any other boot you use in the back also, especially rubber.
An advantage to my release system is that you are not restricted to any one boot. Most skiers have only been able to try the Powershell or the Reflex hardshell boots. This shows many, but not all of the boots I have skied with on my release system. Not shown are the HO and Radar, which are shown on my website.
The various rear half-boots that I have used. I use a very low cut liner, but have also used a full liner with the strap at top.