Edit: the homologation sheets revealed that the very first new age GDB bugeye Sti, called the Type RA (before the appearance next year in '02 of the Spec C), had bigger ports, due to an additional radius smoothing of 0.5mm and removal of the casting shoulder by the valve guide.
To be noted is that in GRB, the intake ports become gigantic by impreza standards, for the Spec C.
The V8 and V9 all have the same big ports.
The V7 were all the same part number for heads, big ports.
Iy was said that the EJ207 for all other markets had small ports. I never bothered to check the part numbers for heads for EJ207 from other markets than Japan.
I've seen before an Australian posting suggesting that their market had big ports at one point for the EJ207.
The main obstacle with those is that many markets had immobilizers very early on.
In UK, I think they may have had immobilizers as early as V7, maybe even the Prodrive P1 in a GC8.
So importing one of those may prove to be an additional headache, even if the early immobilizers were less extensive in their needs than a USDM Sti immobilizer.
But if someone along the importing route drops the ball and msimatches the ECU, keys and immobilizer ECU, that engine will not start.