I ordered my GTI from factory with the 18", picked some BBS-18" on it, later 19" and several 17".
There are some differences between them more obvious when the car ist equipped with a stiffer
suspension (like mine).
ride comfort:
the difference depends on the make and type of tyres. Some more comfortable ones like Michelin
Primacy HP are available in 17" only. When these are compared with a PS2 in 18" or even 19"
the difference is really huge.
When both sizes are the same type and your GTI has OEM-suspension they feel neary the same.
dynamics:
there are many opinions. Some prefer 17" on the track, some like the 18" more. But nobody really
uses 19" for professional driving on a race track with a car like the Mk.5.
The difference in weight between bothe OEM 17" an 18" are just one kg and You won't feel this.
My 19" are 15 kg - that's obviously heavy 25 kg inkl. the tyre. So acceleration is worse and also
turn in.
But it all depends on the tyre again: my Federal 595 RS-R in 17" have a much stiffer sidewall than
my 19" Conti SC3.
Next year I will try a 18" performance tyre like the Yokohama AD08 or the new Michelin PSS on my
8x18 BBS. I'll see then if the performance ist better in 18" than 17".
The 17" will be used in the winter in future. In the moment I drive the 18" BBS with Conti Winter-
Contact 225/40 R18 V.
I suspect it's more the weight you feel. 8 kg Ultraleggera feels much better than cheap 15 kg 19".