With 445HP on the rear wheels, the ability of the rear tyres to transfer power to the road surface is critical. The Alpina Roadster S is factory fitted with 265/30ZR19 rear tyres with 275mm the widest you can go on a Z4 without widening the wheel arches. Obviously that would be no easy modification. The best option to increase traction was to upgrade the Michelin PS2 tyres to Michelin Pilot Sport Cup semi-slicks. The same tyres are used on specials such as the BMW M3 CSL, BMW M3 GTS and the Porsche 911 GT3 RS.
The Michelin PSCs are extremely sticky in dry conditions, improving cornering speeds and further increasing steering feel. Stories about the tyres being hazardous to drive in wet conditions are greatly exaggerated. The Cup tyres actually perform quite well in the wet. Just be sure to stay out of standing water, as it will kill you. Traction in standing water is virtually zero. On the upside, it's incredibly easy to go sideways with these tyres in the wet. Tyre wear is pretty good with the rear tyres capable of 10,000 kilometers of spirited street driving.
The major downside of the Michelin PSC tyres is the substantial loss of comfort. The extremely sturdy sidewalls produce a harsh and crashy ride. Lowering the tyre pressure helps somewhat to overcome this issue. A welcome side effect of the Michelin PSC tyres is a reduction in unsprung weight of 0.5 to 1 kg per wheel.