Model S Performance Edition can hold up to the R8 E-tron WRT acceleration but not with the top speed of 250km/h (180 mph) of the specially-modified prototype. Note that this modification over the standard R8 E-tron top speed of 200km/h decreased lap time from 2:26 to 2:08. That's huge.
The Model S drive train is designed to deliver maximum torque up to 100km/h (60mph) and RPM are maxed out at top speed of 205 km/h (130 mph). You can change the fixed gear ratio but will lose torque doing that.
So in order to compete with the R8 e-tron, Model S must play well in other fields to compensate these 15 seconds. Shed a lot of weight. Tweak aerodynamics and peak power delivery. Make full use of better handling due to low CG.
Still that would be a daunting task and I don't see Tesla wasting a buck on this.