There are exceptions but the general rule is that IT cars are classified with the "standard" wheel size. If you can make a case for getting the 15" wheels added to the ITCS, go for it. Until otherwise approved, the "book" takes precedence.
I got told today that, as far as the GCR and Showroom Stock Specs are concerned, there is no such thing as a 1996 VW GTI - regardless of what Volkwagen says, my car is a "Sport", even if they weren't made that year.
The tech inspectors grudgingly accepted that they were the same thing (from a region officer who's a VW tech) and suggested that if there is an error in the book that I write a letter requesting that it be corrected.
I'm in the same boat as you, I guess: If push came to shove, my car could be found to be ineligible.
K