Hello Adrian,
As I understand it, if you did buy it, and they can’t correct the ‘fault’, then you could reject the vehicle. However, it might well be brought to your attention that you were aware of this issue with the car during a test drive, yet when ahead and ordered one (when incidentally?) in knowledge of this ‘characteristic’. Rejecting a vehicle once bought is hassle, but doable.
You did mention ‘tuning’ in your opening post, but beware that any degree of tuning is likely to adversely affect your warranty, so if it were mine, I wouldn’t be considering going down that road.
Final point - Your actual ordered car may or may not perform the same as the demonstrator, but will the dealer be prepared to allow you to test it before signing??
Sorry to put all the negatives your way, but I would have thought they would now be/have already? registered this vehicle to your ownership, so I would have thought that if you are having any reservations, then you have a very short window to do something about alerting them to that fact.
Good luck with whatever you decide Adrian.
Kind regards,
Gareth.
p.s. I again stress that I may be wrong about this ‘characteristic’ with your particular engine/ model.