You're not thick, and you're not alone in wondering this mate. I can't see it listed for mk5 Golf GTI's on the Forge website either so you may be right. Apart from the massive plate that covers have the rocker cover and the fact it's re-located the catch can away from the charcoal filter (meaning only one design for all engine types), I can't spot any different apart from price. I may send Forge an e-mail to get a low down of the improvements over the original system (which they still sell..?).