To be honest though, advances in suspension, brake, transmission, and tyre technology made later GrpA and WRC cars faster over the stages than the GrpB cars, even though they were a fair bit down on power, though not a huge amount between a early 90s GrpA car (way more powerful than current WRC cars) and a rather low powered (in comparison to others) GrpB car.

For example....

1986 Rally of Sweden
Distance: 1670 km including 558 km over 30 special stages
Jean-Luc Thevenaud- MG Metro 6R4- 6:07.08

1992 Rally of Sweden
Distance: 1674.05 km including 571.19 km over 31 special stages
Stig Blomqvist- Nissan Sunny GTI-R- 5:26.09

Unless im mistaken, that makes the GTIR significantly faster than the 6R4, and over a longer distance.