Not a single Group B stage record still stands does it? They were epic, but they were just power with very little control. Cars are now going faster.
Are the stages
exactly the same? As far as I know, very few stages are the same now as they were in the mid 80s.
Maybe a modern WRC car is faster than a Group B car on any rally stage, given that they are much better in almost every way, but the comparison would have to be on identical tracks. Preferably also in identical conditions. Perhaps, for example, Group B drivers would have been faster without spectators being on the track and grouped at the side of the track in the most dangerous positions.
A modern WRC car will spank a Group B car in everything apart from power to weight ratio, but a Group B car will spank a modern WRC car in power to weight ratio (e.g. Delta S4 has 2.2 times the power to weight ratio of a Focus WRC and that's with the stated figure for a conservative tune on the Delta, which is less than half the power it was capable of). I think which would win would depend on the stage, with the group B brute forcing a win if there were enough straighter parts.