Not sure if you purchased yet but...
I test drove a bunch of GTIs before buying my CPO and I noticed a few things.
Drive it on the freeway. Don't remember what it's called but the rubber seal thingy around the door tends to crack and wear out and you can hear a whistling if you drive it above 60 mph. Heard that in a few cars.
Check in the trunk for all of the crap you're supposed to get like the spare, the tool to actually get the lugs off, the jack to get the car up in the air, etc. All that stuff has cut outs in the back and if you see outlines for stuff but something isn't in it's place chances are it is missing entirely. This was pretty common.
If it has over 10,000 miles or more make sure they did the service on it you're supposed to receive for free every 10,000. Doesn't seem like a lot but one dealership didn't want to provide this service to a GTI I was looking at even though it had 12,000 on it and they considered it long and gone. Maintenance is important so make sure it's been done or that they'll provide you with any maintenance it has missed with no hassle.
If you're not buying certified (CPO) make sure they aren't charging you the added fees for it being certified. I was talking to a service manager and it actually adds about $800 to the car's price when it's certified. I think this may be the cost of the CPO warranty but not entirely sure. I personally wanted CPO because the warranty means a lot to me but you should be aware.
Hopefully some of this helps. Good luck with your purchase