That's a seriously nice Mk5 Steve. Personally I'm not too keen on the lime but that's a very minor criticism and purely down to each individual.
I really wish the dash came flocked as standard.

Back on topic, I do concede that you are right. At the end of the day while a Mk6 is a good compromise, it is not a fully set up track car. For such a project, there are plenty of options that are perhaps a bit more suitable - or at least ready.
Just to add though, I did also have the pleasure of taking a Clio 197 Cup - I think it was actually the 200bhp edition - around the Ring. At the time I thought it was great as the steering, gear box and handling were all very sharp and precise and the whole experience felt decently quick despite the definite lack of grunt (particularly on the Ring's many uphills).
But then taking my Mk6 around the same track - yes it's not as "pure", but the GTI felt far more forgiving and better suited the varying track conditions. The brakes definitely weren't as strong, but on turn in the chassis offered IMO just the right amount of play while remaining very stable. The suspension was also supple enough to soak up the bumps quite well. (Although I know the Ring is quite unique in that a super stiff setup is actually less preferable to a more road based setup due to the number of bumps and camber changes).
So all in all, I agree with you Steve. A proper track car will always be better on the track but to the OP, don't right off your GTD. It's certainly worth a go in!