I had one on my insert. I'd highly recommend it. Even though our insert was shrouded, a lot of heat has to go straight up. I used sheet metal with rock wool on top
I had very little room to get a block off plate in so I just stuffed mine with Roxul around where the liner comes down through. Works just fine.
I have a free standing stove in the fireplace and I have ran it first with nothing, then with Roxul stuffed around the liner where it passes through the old damper, and finally with a proper insulated blockoff plate. I was very unimpressed with the stoves performance with nothing in the hole. It was obvious that there was a lot of heat lost. Just stuffing Roxul in the hole seemed to help quite a bit. But once I installed an insulated blockoff plate it was soon obvious that it was worth the trouble. I did notice that the fireplace sidewall brick got pretty warm after running the stove for a few hours. What I did was to just put in some freestanding metal heat shields against the sides of the fireplace. There is no insulation them but there is a air gap behind so that air can freely flow. That has reduced the brick temperature a lot...so it figures there has to be more heat available to the house instead of lost to the outside wall. Each step along the way improved the performance of the stove...as far as the heat output to the house from a comparable load of wood
Gotta have the plate. Without the plate there is a slang term for the install. It's called a slammer. Term comes from installers slammin a stove in quick and cheap to make more money. The plate is the correct way to install. Also provides the opportunity to loose fill the chimney way.... this keeps the pipe hot and increases draft....
I have to disagree with your comment on the "slammer". Slammer is an insert with no liner attached, just slam it in and let the smoke find it's way up the clay liner. I burnt mine like that with my old "Fire Boss" insert for many years and always had much more creosote than now. No disrespect.
