I've always wondered what percentage of GTI owners are 'enthusiasts' rather than those who are just buying a top Golf version.
I reckon a small percentage. The whole point of the GTI is that it's meant to be everything to everyone. It's appeal is so broad and enthusiasts is a small part of that spectrum!
....I agree that it's probably only a small percentage. However, what you describe as "the whole point" applies to the Volkswagen Golf, not the GTI specifically.
Volkswagen = Volks Wagen and translates from the German to People's Car. The original was Ferdinand Porsche's 'Beetle' which was enthusiastically supported by and only happened due to Mr Hitler. The Golf model was originally introduced as a successor to the Beetle and was designed to embody many of the same values.
In ~1976 the GTI was merely a variant of the Golf and not originally intended to go into full production. So it's the Golf and not the GTI which may be described as being meant to be everything to everyone and with such a broad appeal.
I see the GTI as it was originally conceived by the small splinter group of VW staff, as a sporty version of the Golf and having a higher status as a result. So, a GTI owner
OUGHT to be an enthusiast, although this is not the case often enough, IMO. It is the genuine enthusiasts who carry on the GTI tradition - Drivers like us who frequent forums and go to shows and meets etc as a community.