^ That's the way I do depreciation too. I care what the car has lost me, after discounts and what I p/x or sell it for, not the used sticker price (which is about 20% higher than p/x).
Even so, every time I have looked at changing cars, I've never seen a performance Leon's discounted price corresponding cheaper than the Golf to the tune of cancelling out the extra depreciation (to p/x value) it would suffer over and above the Golf.
Maybe it's my local dealerships having not been too competitive in the past, but I'd want a Cupra to be at least £3k cheaper than the equivalent Golf at purchase to cover it being worth £3k less at 3 years old in p/x.
Never happened for me, nor have I been wanting a change at a time where 0% finance was available.
Best depreciation relevant to price is likely to be the R when I trade in in a few months time - £32k car, bought for £28k and £16.8k p/x value at 4 years old, less than £3k a year real depreciation!