I'll try to explain why a period tyre is more suited to an earlier car than a more modern one, but please do remember i am just a tyre dealer with a grade 4 CSE in English not an eloquent engineer turned journalist.
More modern tyres have a relatively wider foot print compared to the width of the tyre than a classic tyre, the lower the profile the more that happens, which means they are less progressive, ie the brake away of a modern tyre is more violent which makes it more difficult to catch. and a modern tyre is not developed to deal with the car drifting at all. modern cars are meant to have ultimate grip. However, in this period cars moved underneath you when you cornered hard, which is a big part of why they are fun to drive.
a Classic tyre has more rounded shoulders to cope with the roll in corners.
Classic cars (here's where my knowledge of specific Porsche models is limited) have a situation where the tyres stay parallel to the side of the car, where a modern cars have a set up where by when you have heavy cornering forces as the suspension reacts it has the effect of adding adverse camber which makes them more suited to the square shoulder of the tyre, by keeping all the foot print in contact with the road. if you fit a modern square shouldered tyre to a car with no negative camber it climbs onto the shoulder of the tyre when cornering and then brakes away more violently. heres a rubbish picture i drew:-
What that is demonstrating is that in the lower picture you will notice a picture of me smiling as i am enjoying the progressive handling of my over exaggerated rounded tyres. as the car rolls when cornering i maintain a reasonable foot print however the car above with a rather sad gentleman has climbed up onto the over exaggeratedly square shoulders of is modern tyres and will suffer with unpredictable handling and a car that goes round a corner like a 50p piece
When you look at a modern car, that has the type of engineering in it that a modern tyre is designed to suit. if you look at it parked up, with steering lock on, you will notice that it's front wheel is at a funny angle, sort of tilted back, that is called caster and that is put into a modern car to help overcome some of the derogatory effects of modern wide foot prints. Then the derogatory effects of this Caster and wide foot prints gives horribly heavy steering and this is then overcome by modern fangled very clever power steering.
I don't know if i have managed to make any sense here. However may i suggest that, regardless how legible my illiterate explanation is, Porsche them selves have decided that these are the best tyres you can get to suit cars of this period and they have given them the N4 homologation. they should know.
Granted if you are racing or you have modified you car (probably by dialing in some adverse camber) then more modern tyres might suit. But for a proper old car, tyres that are developed to compliment old cars are better.
here is a short film of me in my pre Porsche, Porsche; demonstating that even when you have one of the worlds most progressive handling cars in the world you can still make a complete bollox of it (am i allowed to say bollox?)
https://www.youtube.com/watch?v=dT0Y1wOMnuY
