[asperformance]

as a 97 STi your car will have left the factory with the 12mm under-bucket shims as fitted to classic STi's from 1996.................
what your describing at 30mm is equivalent age std WRX/UK over-bucket shims - seems fairly easy explanation/fix as your car has obviously had its heads swapped for std ones at some point in its life??

we have a lot of shims in stock, cant see why an engine builder/parts supplier cant offer the same???