Wuz, I'm quite sure that Patsy was referring to the upgraded pipe on the WRX when fitted with the PPP, that has been quite correctly quoted as being changed to a blue silicone unit, rather than the standard black hard plastic unit. Possibly Patsy didn't notice the part about it being a STI, and not just a WRX.
And TBH, if I was looking at 2 cars, of similar price, and one had a PPP, then I know what I'd put my money on first.
Mike, all cars are listed as PLG - Private or Light Goods - it's the exhaust emissions that decide what tax bracket, and IIRC, if not stated, then DVLA will base it upon a UK based car or put it in the highest category.