I'm not really sure how the set up is on a regular zetec but on the SVT if i replace my cat with a high-flow cat it moves me into STX.
what did you replace your cat with? do you have a high-flow? ORP?
this means i could run a high-flow cat in STX and also add a LSD.
IMO the Focus can't hang with the BMW's or WRX's in STX.
The Focus goes to FSP rather than DSP.
Edit: they may not know or care, but you're cheating yourself by not following the rules.. believe it or not, the PAX factor and car classes are pretty darn accurate and help level the playing field.