Really the KR and SS are all extended versions of the current GT500. The only difference are bolt on's, suspension, tune, etc. They all use the same 5.4L iron block, heads, tranny... There are plenty of people that deck out these cars... TVS s/chargers, KB, some have whippled, CAI's, exhaust... there are a few running a twin turbo kit... the options are endless.
My "stock" Shelby is currently getting a new drive shaft. It will have mods someday.