The Guaranteed values in my opinion on GTD are way to high and I can foresee quite a lot being handed back because they will have no equity or be in negative. Lease and contract hire companies are throwing these cars out by the dozen because GTD/GTI allocation is way more available than it used to be.
£15,000 residual on a 5 door dsg in 3 years time 10k per year........ who's going to pay £17,000 retail price for a 3 year old one when you can get a new one out the box for only £8-£9000 more?
I was quite surprised to see my 27 month old Scirocco 170TDI GT with a sticker price of £18k at my dealership after trade-in. I thought Scirocco prices had softened a lot over the last year or so. I have always thought that used VW prices are generally stupidly high (especially GTI/GTD), the main reason I buy them new. I always wondered why someone would pay 75% of RRP for almost a 2.5 year old car rather than 90% of RRP for a discounted new one.
As per your comments, it seems crazy, but those prices do seem to be the norm in my locality. £15k in part-ex/GFV on a GTD does seem to fit that model, but you're the dealer - what do 3 year old MK6 GTDs go for right now at yours? That might be a good indicator, even though the MK7 GTD is significantly better than the MK6 and the MK5 170TDI GT/GT Sport was never really a GTD in the trim/appearance department.
GFV does look great when you can get a GTD for effectively the same cost as a GT 150TDI over 3 years because of the clearly better GFV (56% vs 47%), makes me wonder who would get the GT rather than the GTD on a 3 year ownership basis.
The norm for GFV is to very slightly underestimate true value so you have "equity" for the next car, as a bit of a hook. VW will have cocked up big-time if they don't have that "hook" and you give the car back, free to pursue any other marque of car without thinking you're chucking away that usual equity.