When I found out that the 981 Sport Techno wheels were half an inch wider than the Carrera S wheels that come stock on the GTS, my first question was: where is the extra 12.7mm distributed?

Carrera S wheels on stock 981 Boxster / Cayman GTS:
Front - 8 x 20 ET57 - 235/35 ZR 20 (88Y)
Rear - 9.5 x 20 ET45 - 265/35 ZR 20 (95Y)

Sport Techno wheels optional on 981 Boxster / Cayman / GTS:
Front - 8.5 x 20 ET57 - 235/35 ZR 20
Rear - 10 x 20 ET50 - 265/35 ZR 20

Boxster / Cayman / GTS Sport Techno Wheels:
Front: The offset on the Sport Techno front wheels is identical to the offset on the stock Carrera S front wheels, therefore, the increased width is distributed evenly between the inside and outside of the rim.
The inside lip of the wheel is 6.35mm (1/4 inch) closer to the longitudinal center of the car.
The outer lip of the wheel is 6.35mm (1/4 inch) farther out (wider) in the wheel well.

Rear: The 5mm of additional offset on the rear wheels give the final result:
The inside lip of the wheel is 1.35mm closer to the longitudinal center of the car.
The outer lip of the wheel is 11.35mm farther out (wider) in the wheel well.

I'm very happy to find that 90% of the rear rim's extra half-inch of width is distributed outward. I hope this helps other people in search of this information.

 

Rear: The 5mm of additional offset on the rear wheels give the final result:
The inside lip of the wheel is 1.35mm closer to the longitudinal center of the car.
The outer lip of the wheel is 11.35mm farther out (wider) in the wheel well.

Can you clarify this?
When I punch the Width and Offset into a calculator compared to the other 20" wheel, it says:

Compared to your existing wheel, this new wheel will have an inner rim which is 11.4mm closer to the suspension strut. The outer rim will poke out 1.4mm more than before.
That seems opposite of what you are saying?