The ultra-hot mutation that causes peppers like "ghost" to produce capsaicin from the pericarp instead of the placenta alone allows for breeders to increase the capsaicin to berry ratio by increasing surface area and decreasing berry flesh thickness. This is the key to the development of peppers like the Reaper and others.
I will have more information after a few test crosses.
I have learned that the mutation is in the case of Naga peppers an allele from Capsicum frutescens that causes increased oil production in the endocarp. Some of the hot thai types have the trait but when the trait is combined with C. chinense genes the result is able to reach a higher SHU and dry weight value than C. frutescens...