Sorry to drag the topic further of topic, but HistorX includes the very cars that set the real life times - the TC65 Cortinas belonging to Warmenius and Gustavsson. And the tyres used in TC65 try to emulate the tyres of the period including grip level - competitors have to use Dunlop vintage racing tyres.
To try to get back on topic, try to find some real life laptimes for the 2013 season. While laptimes depend on other factors such as mass and power, these are usually easy to check and get right.
I don't know how the cars of todays historic racing compare to their counterparts back in the day spec-wise but if i watch recent vintage racing videos, the Cortinas don't lean & slide nearly as much as Jim Clark does in this little clip:
If you watch the replay of a fast Cortina lap in HistorX it looks
very similiar to that
"Swinging Style", it's pretty natural to drive that car in a little drift, again contrary to how i perceive the cars movement in modern videos of the same car.
I'm inclined to think laptimes would be different too, if the movement of the car is different.
I also agree with Rupe that ultimately it all comes down to fun but in most cases
more real = more fun (at least for me), since you get more joy of a car if it does what you expect it to do.
Regarding GSC, i remember when the Minis were about to come out the guys at Reiza showed a telemetry comparison between the real life & the sim car which was pretty much identical, i'm no physics engineer but that has to be 1:0 for Reiza.
I don't think you can compare GSC with HGTTC, except for the Opala which is a car from the end of the 70's and if you compare it to a '71 Capri 2600RS it certainly doesn't feel as if it had much more grip, especially considering there's almost 10 years between those cars (I'm not really sure it has more
at all, haven't driven the Opala for a while but it doesn't strike me as very sticky in my memories).
Obviously there's gonna be a huge difference in grip between a 40 year old Mini on narrow street tires and a modern, race prepped one on slicks
.