Thanks for the input guys.
I purchased this grill from a very reputable seller here on NASIOC who took it from a full front clip that he had imported. I suppose it is possible that an aftermarket grill was fitted in Japan. However I have seen other images of non-sport grills very much like this on JDM STis. I also received the grill side pieces with the "vent" cutouts as well as the V7 STi scoop from this clip and they appear to be authentic.
The mounting holes are in exactly the right places, just too big. I am almost sure they have not been drilled. There are no signs of drill marks and the plastic is built up around the holes as you would expect for them being molded that way.
I'll keep looking.