There's no escaping the fact that the GT pack considerably helps when selling and also improves residuals. The big question though, is how much of the £2500 cost do you actually get back when you sell?
Whatever % you lose from the initial cost of the car will also apply to the £2500
Therefore if a £30k non GT maybe be worth £15k in 4 years time a GT pack car would be about £16,250. Well, thats how I would view it if I was purchasing a zed.