In the end I also decided against all-season tires. It's a car with tremendous performance... the tires have to be able to withstand it too! So only the best material. The tires are your only contact with the road, old adage.
Regarding winter tyres, especially for the GT version where performance counts a lot, a few days ago I saw a comparison test between standard winter tires (were Hankook) and winter tires specially designed for electric cars (Hankook iON).
The performances were much better with the iON (acceleration, braking...). Do you know if Continental, Michelin or Pirelli have developed similar winter tires for electric vehicles?