^^^thank you for your insight and knowledge.I think maybe it is worth shopping around a bit for the Lightweights.
305/30- 20 would fit ok on 11" but I believe 11.5" rim is ideal.
If you do a search on here David Lemmens has 8,5 J x 20 ET49 + 11,5 J x 20 ET56 991 Carrera S IV wheels with 255/305 tire sizes on standard suspension set up.
According to Wheel Offset Calculator | 1010Tires.com Discount Online Tire and Wheel Store
20 x 8.5 et 47 would give you + 16mm on the standard Carrerra S wheel so you will be getting close to the limit, Seb Vittell (Manthey Racing GTS) runs + 17mm with no issues but has more camber.
20 x 11 et 45 would give you + 13mm on the standard Carrerra S wheel which would be fine. Seb Vittell runs ET40 so extra 18mm with spacers.