Jomama45,
For all of my masonry work that requires refractory mortar, I have been using
Thermal Ceramics Smoothset mortar (that is all I could find in Phoenix). How thick is the Heat Stop II coating?
Maybe a 1/4" thick total for both layers combined. I've never heard of the refratory you used, but as long as it's NON-water soluable, it should be fine.
I have also seen some people who don't make curved throats on modified Rumfords; they make them straight. Any thoughts on that method?
I just re-read & realized that you're only using a gas insert. An angled throat should be sufficient for what you're doing. I would personally be aprehensive to do it though IF it were me. It's too easy to convert the gas log set into an igniter, and then try to burn wood in it. Odds are someone down the road may want to do so, and they may be unpleasently surprised to find out the FP doesn't draft decent. I'm sure with some creativity, you can figure out how to pour a curved lintel to place at the front, and just run the back & sides up with firebrick.
Do you have any more pics of your fireplace? It looks great in the ones that I have seen.