Chamber tech 2000 used as firebrick backing?

I have a fireplace that is in the center of the home. Its back is the wall that's just the right side when going downstairs (in other words, the back of the fireplace is just the wall that you can run your hands along as you go down into the basement; that wall stops at the floor level of the first floor, it's empty space after that). It's a cinderblock chimney, and that wall on the other side has gotten hot to the touch when I have fires for a long time.
Now, there's no combustible material in there, it's just the firebox (firebrick back), then some more masonry (could be a second layer of cinderblock), then plaster smeared on the cnderblock to form that "wall" on the other side. I measured out theings as best I could, and from the back surface of the firebox (the surface you put your hand on when reaching into the fireplace to touch the back), there's about 6 - 7 inches to the surface of the wall (running your hands on it as you go downstairs). So that's 2 inches of firebrick, then some masonry or whatever, the cinderblock that's used to be the big foundation of the chimney (that goes all the way down to the floor in the basement), then plaster that's painted.
So, we're having the back rebuilt, and the latest chimney/fireplace guy said that to address my heat concerns (and since we only have a little space to work with), we could use those 1" thick firebricks (1/2 the thickness of the standard size), and parge the remaining space behind it with this chamber tech 2000 stuff, which is apparently so kick-ass at insulating the heat that the guys says that 1" of that stuff, w/ 1 inch of firebrick, is better than the whole firebrick.
Has anyone had any experience with this stuff? I can't find any info on it except what it says on the bucket (which is what the guy said is all he knows; he hasn't really been able to verify the difference between this stuff and firebrick in terms of actual measured heat protection), and no reviews online, etc. All I could find was a post in which someone said this stuff is mean tto seal cracks, not actually be used as entire flat wall of surface behind firebricks.
Any opinions on this? Should I really do a layer of this stuff behind 1/2 thicknes firebrick, or should I just do the whole firebrick?
Any experiences or expert opinions on this would be GREATLY appreciated, and thanks.
PS: I know all the local codes and general parameters for a safe firebox,we're just working with what we've got, because an insert is out of the question. Also, I'm positive there is no combusitible material within 10" of any surface of the firebox; it's all plaster smeared onto cinderblock (or brick) well up into the 2nd floor. My fear here is heating up the cinderblock so bad that it crumbles, or cracks, or whatever.
Thanks again in advance, and for taking the time.
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