We just developed a masonry heater performance calculator to forecast the heat gain over time for a given load of wood and the various facing parameters. It was recently put on the international Masonry Heater Association news website:

Masonry Heater Association News

Anyhow, if you have a masonry heater and IR temp gun, we would welcome your wood load, facing parameters and temp/time data of your facing surfaces to validate and perhaps further refine this calculator. Thank you.