TA – you were not part of the club the last time we tried super skinny tires, (e.g the 2014 Trasn-Am). In that case the spec was BRM S-117, shore-30-22mm-Dia x 9mm-Width. Results were brutal, altho to be fair those cars were generally Heavy.
I favor having “open” tires with a max width of 10mm; contingent on them fitting under the wheel-well Lip/skirt; and they need to look reasonably “scale” and period-correct for the car.
If that means your cars need 6mm wide tires; Goferit! If that means 10mm tires look appropriate and fit in the wells on John’s car – fantastic.
IMHO the term “Reasonably” should be used with lots of judgement; the more “rigid” we get on this series with specs, the more we deviate away from its theme of “a builders series” and a series where “Concours d’Elegance” and innovation are weighted higher than most other criteria for qualifications.
"... get on your bad motor scooter and ride!"
Sammy with Montrose