I'm going of Benny Woosters numbers, he got his down to 1280 kg, with gtr kit, and pretty resnobale amount of weight savings, but his cage isn't hugly extensive. Short of carbon, titanium, and cro-mo cage, it won't be any lighter.
you can't look at a group a car for weight numbers. Fristly they had a pretty basic cage compard to todays standards, and things like most of the suspention being custom milled from alloy, and use of lightweight materails everywhere.
if you want to race for long stages/runs you have to add weight in to make it reliable, bigger radiator, oil cooler, bigger brakes.