Soldato
It isn't so much the rear wing but the whole aero design from the initial contact of air over the front wing to managing the air through the rear diffuser. The idea with using a wider slick tyre is to increase the mechanical grip so the cars could corner faster and follow each other. These cars are capable of slipstreaming very well, but when it comes to a corner by following another car you're losing the air over the front wing which affects the whole balance of the car. The tyres are no longer performing to their optimal level and you're going to lose grip dropping your speed or excessive wear of the tyres. Would either need to make the front wing a standard part limiting the flow over the rest of the car so it's relying on the mechanical grip, or introduce ground effect.