I'm getting a Supersprint system fitted this month and asked about a decat, the tuning company who's fitting and supplying it recommended not getting one as it doesn't improve the power in any way and is a hassle for MOT's. Straight through isn't the way forward either. Decent system, quality induction kit, give it a good service with quality spark plugs, etc, and she'll be running sweet as a nut and will be fine enough until you're ready to make bigger upgrades to a Schrick or a charger, etc. Engine wise, VW didn't do much wrong, so there's not much to change. in my opinion anyway.