Never mind, found the answer...
SEAT has also confirmed details of the wheel, tyre and brake package fitted and to be included in the Performance Pack. The record car had to run on Mk2 Cupra 19-inch wheels to clear the Brembo brakes, which run four-pot fixed calipers clamping 370mm discs at the front. Standard brakes are single pot sliding calipers and 340mm discs, the rear brakes the same on both. Homologating a new 19-inch wheel for the customer pack is one of the reasons it's not available yet. Wheel offset and tyre size (235/35R19) remain as before but the Bridgestones or Pirellis of standard cars are replaced by the Michelin Pilot Cup Sport 2s. There were no further modifications to the chassis, engine or any other parts of the car.