Am I missing something here?
Don't used GTs cost a little bit more than GTAs as they are generally newer? So, therefore, you can carry out the suspension mods to a GTA and still be quids in.
GT JTDs are bottom of the GT range and so their used values are reflected by this, yes because GTAs are older you could buy a cheaper GTA but if you are swapping car you'll go for the best you can afford and you'll never get what your car is worth, the later the car the more issues fixed like the brakes etc.
Also, don't diesels require to have their cambelts changed at more regular intervals?
And, as has already been discussed, chipping a diesel doesn't aid fuel economy and (correct me if I'm wrong) makes it go through clutches at a more regular rate.
As mentioned earlier if it effected my economy by more than a couple of % i'll be surprised, mine was remapped from almost day one, after I'd done a couple of k running it in.
Regarding clutch its funny you should mention it I think mine had its first slip today coincidently
, hard to say what life is common, I have heard of them go early, but I've done a **** load of competitive sprints and trackdays, plenty of hard launches and still managed 60k out of mine, seems reasonable to me, its coping with over 265lbs of torque for a couple of thousand rpm.