Up or down ?
Model would have to include new variables for gravity (fixed, at least at sea level) and angle of the hill, no doubt expressed in radians to make it more difficult for me to get my head around. Gearing considerations / change points could I think also become more relevant too, but I think the VW engineers thought of that.
I just developed it out of intellectual curiosity as a way of back calculating what power my hybrid engine had without taking it to a rolling road, then tested against some other cars where data was already known. It seems a half decent fit for an amateur and produces some counter intuitive figures.
Hill climbing ability used to be quoted for VW's. The brochure I have for the Aircooled has the values. I haven't seen any for the WBX and interestingly while the UK brochures continued to have performance data, the German brochures did not. Perhaps related to the fact that in Germany if your new car could not meet the quoted performance figures in the brochure you had rights of rejection. Something like that anyway - or maybe just a pre-internet myth.
At 70 mph around 80% of the power requirement is being used to overcome aero drag and you only need ~ 50HP to maintain that speed. Even a 1600 CT has just about that many
I imagine in a DF you may have to work the gearbox a bit more to keep it within the narrower available rev range, but as VW themselves said "throughout the lower engine-speed range it produces the same willing flow of power". The DF will be slower than the DG, but when slogging up a long hill the difference may be less than imagined.
I think you mentioned perhaps being down to 40 mph on a m-way hill before. That only needs ~12 hp on the flat, can be achieved in 3rd, 4th and 5th gear, and in 3rd gear that corresponds to 3500 rpm which is peak power in the DF of 60HP or 65HP in a DG. Expressing it another way there's a further 48HP or 53HP available to overcome gravity.