I did something similar but with different materials. The first material was essentially shop floor 3/8" rubber cushion 3-M glued to the inside of the engine compartment. That lasted a few (maybe 7?) years and then the glue let go. It was a great sound deadener, though.
The next stuff was more like Dynamat, black, but thinner. It's still in there, isn't as good as a sound deadener but the glue on it ain't lettin' go for nuthin.
Those tar boards would be pretty slick in a Speedster IF you intend the engine compartment to be black and IF they really stick - i can just imagine ingesting one into the fan at 3,000 rpm.
If you're doing something like Rich is doing with a detailed and color-matched engine, then a color-matched compartment is the way to go, but the noise level will be higher (Vanity, they name is "car shows")...