I'd say all season tires are BS. My cayenne didn't stop during winter time with those all season tires recommended by Porsche. My buddy almost fell down the cliff too because the car didn't stop.

I'd say use the 20"s for summer and 19"s with winter tires for winter, its the best option, it only takes 5mins to swap those wheels

__________________
04 Porsche Cayenne S SW(Retired): 22" HRE 645R, Frozen Rotors, Techart Sport Muffler, Twin Tailpipes, Fabspeed Bypass, Techart Turbo Full BodyKit, Magnum Hood, Headlight & Taillight Covers, Eibach Springs, BMC Airfilters, Tequipment Roof Rails, Roof Strips, V1 Hardwired, Silver Hood Crest, Clear Marker

08 BMW X5 3.0si AW: 22" HRE 996R, GP Thunder 8500K H8 & H11, ACS Full BodyKit, ACS Sport Muffler, Racing Tips, M-Tech Flares, Running Boards, CF Roundels, H&R Springs