I read somewhere that this infill methods will have approximately r-2 per inch of thickness, compared to Owens Corning rigid foam insulation which has r-5 per inch.

Here's some r-values fro comparison:
Extruded polystyrene 5.00 per inch
Expanded polystyrene 4.00 per inch
Polyisocyanurate & Polyurethane 6-7 per inch
Blown in Cellulose 3.70 per inch
Fiberglass Insulation 3.14 per inch

compared to the estimated 2 per inch of a light clay infill.

However, R-value alone doesn't tell you the whole story. Clay also acts as a thermal regulator, meaning it holds on to heat. That means that heating systems don't have to run as much to keep a building in a comfortable temperature range. However, a light clay system likely won't do this enough to be of much consequence.

One thing about charcoal that I did not mention earlier that could be a possible drawback is that it is highly absorbent. However, charcoal will absorb a certain amount of chemical and then simply stop absorbing. To my knowledge, it does not then diffuse those chemicals into the air later. It likes to hold on to them. Which might actually be a good thing, the wall might absorbe the various toxins that are floating around in a home when it is first built such as paint and glue solvents, and so forth.

This weekend I plan on making some experimental bricks out of a variety of materials such as Charcoal, Wood chips and shavings, straw, and maybe waste paper. I also plan on subjecting them to a number of tests once they are dry, like testing their fire resistance, conductivity (not very scientifically though) and how well they hold up to water. I'll post my results when I have them.