Jason valued my 3dr at Ford Fair last year, very thorough, very honest and actually valued it at more than I valued it at myself.
I don't see this argument that Saff values are being 'held back' to pander to 3dr & RS500 owners, they're all different cars in their own right and all have their own markets and buyers, so values of Saff's shouldn't have any impact on 3dr/500 prices, and vice-versa.
It's been said before, but you will never ever get the full value of any mods back when you sell a car, so your valuation of it shouldn't be based totally on what you've spent on it.