I'd be inclined to agree with Worzel on this. Prodrive stickers don't mean a thing.
Its probably a PPP'd STI so 305bhp on paper (more like 290bhp when they typically went on the rollers) and the chaps got a PPP certificate which has confused him. The WR1's were the only blobs with 320ps (315bhp) as far as i'm aware. The confusing bit is that he's got a number from somewhere.... no 6 of 125. Wonder what thats come from?
There were certainly special editions in the blobeye era (WRX300's for example) but i can't think of any that were STI based other than the WR1 or the S203 (latter of which is nothing to do with Prodrive).
Still, always open to being proved wrong