To quote Motortrend, "it took [the Mclaren team] about a day to get the prima donna sorted out." After that Pobst was able to get a 1:27.62 out of the car. which isn't much faster than the GT2RS...
I have heard customers complain about this car being very prone understeer. which makes it a lot more safer for a car on the road with this much power. Same with the Pista. But if you want the ultimate lap time the car needs to be re-setup to get the most out of the car. This might be why some times the lap times can have such big variances.