The problem with the original and revised gainmaster power ratings is that sirio made them up,
some people get away with low duty cycle modes SSB/AM at 500w or more on a stock antenna,
but less than 300w of long winded fm will have your vswr rising often untill your amplifier leaks smoke,
Gainmasters handle about 200w continuous carrier without upsetting the vswr on long overs,
the issue is more than the capacitor not cut neatly or been too small,
Gainmasters use bieffe rg58 size foam dielectric coax,
http://www.bieffecavi.com/index.php?lingua=eng&pag=prodotti.php&categoria=coassiali50
ok for 300w continuous or providing vswr is very low 600w with the coax getting pretty warm,
the same power will cook rg58 size coax if vswr is very high
only the coax from the gainmasters so239 connector up to the matching stub has low vswr, the coax between the stub & feed point has high vswr & should be derated accordingly,
the stub also gets hot as hell with 300w fm.
simonthewizard did say something about a higher power version maybe in the pipeline.