I don't have the applied model codes for any 2.5RS. All I have to go on for those is the actual VIN, which to my knowledge, were all GM6 or GC6; where 6 is 2500cc AWD.
It's curious, but the Forester XT and the GC/GM RS both share the same engine code in the VIN.
I guess GC8 would almost certainly be the applied model code because when we are talking VINs, there was no such thing as a GC8 in the US. It seems to me that the term GM6 and GF4 were incorrectly derived when the GC8 term was thrown around originally.