I hate the merged forums, so many people who don't play nice.
We all have our opinions, here is mine.
18"s on the GT are a challenge, but no more than any other tyre on a GT due to amount of "using the force" required to judge where the corner is.
I have done mine in, but only at the back, and only in a car park where there was a high kerb that was not marked on the "lanes" to the ticket machine
In normal road parallel parking it should be no problem, but practice makes perfect, moving the mirror is your friend.
The ride on 18's is WORSE on a GT, which is what the original poster has, opinions from 156 owners may be valid for 156 but does not help the original poster with his GT issue.
typos1 if it were my money, I would stick with the 18's, get em repaired, try to tune your spidey-sense for parking, and perhaps carry a sledgehammer for revenge on any kerbs that get you.
If you have a JTD it sits higher than the petrol models, if you stick 17's on you will regret it, and then have to spend a lot more on the suspension.
Just my two cents...