I know that there was a non-standard disk format called "Hyper Density" - a term coined by the author of the well-known and quite popular "Scheibenkleister" book. It clocked the FDC with 10 MHz (not 16 MHz, as a high density kit would do) and that was switchable by PSG port 6 (as documented on the "Kleisterscheibe"). This format allowed 13 sectors / track, the downside being that it was not readable on computers without that mod.
You can find a kit that supports both "hyper density" and "high density" here:
http://www.wrsonline.de/hdkit.html. The
hyper density switching is done with the PSG, whereas the
high density (1.44 MB) switching is done via a signal from the drive (as in your high density kit).
EDIT: That 10-MHz/"hyper density" mod is mentioned in the FastCopy Pro documentation, as well.