The idea of cutting the header, little by little will only work if the header is too long to start with!
If you have around 10" from the end of you muffler to the glow plug then I guess the optimum rpm would be above 20000rpm (depending on engine timing etc). So I would say in your case you need to increase the header length and then see what happens.
The MMVS "tuned mufflers" have a tube inside such that you get a "folded pipe". The pressure wave will travel on the outside of the inner tube to the front of the muffler and the back again. The stinger needs to be smaller than the inside tube in order for the pressure wave to travel back into the tube. Here is a picture of a cut-up MVVS "tuned muffler";
At the "SPAD-to-the-bone" site they mimic the MVVS construction, as I have understood it;
