I'm sorry if I'm a bit "obtuse" on the regs, I just wasn't clear where the confusion is. STCS does not specify gas or diesel other than to refer the user to GCR 9.3.26 for fuel requirements, and 9.3.26 starts out with something like "permitted fuel is defined as gasoline or diesel fuel meeting these specs" blah, blah, blah. Ergo, diesels are allowed in Super Touring and must meet the same prep regs.
Now, we have a request on the table for an alternate turbo for the older 1.9L diesel, and we're trying to get specific turbo information from the guy in order to address that. If someone is really sharp on these engines we'd consider some kind of alternate turbo for the whole range, we just need specific information to list on an allowance line. I don't think we're at a point where we're willing to let turbodiesels run without restrictors, but given their limited RPM range I don't see the TIRs having much of a performance affect on them anyway (except the wallet).
Bring on the TDIs! If you want some alternate equipment, send us a note with what you're looking to do with supporting info on what will result, and be as specific as you can.
GA