Agree with the above. If someone wants to buy a GTI with a view of modding it, it can make sense to buy one that's already been done, so long it's sensibly priced and been done properly.
I wanted a standard ED30 and bought what I thought was such a car, only to discover it had been chipped. I did think it was a bit feisty during the test drive (spinning up 3rd for fun in the dry) but just figured that's how they were, being a bit fruitier than the regular GTi. So yeah, if you test drive a 'standard' GTI and it's a bit of handful, you can bet your bottom dollar it's been tweaked.
IMO the standard mapping is boring. Not enough boost and too linear. The rest of the car is spot-on imo, so I avoid aftermarket suspension personally.
I've sold modified VWs in the past and lost a bundle, and I didn't get many calls, so give it some thought before selling.