As far as I know, the 198mph top speed was "claimed" by Volkswagen.
This new GT weighs over 5000lbs, and I don't know any tires on the market today are actually capable of the claimed 198mph.
But the 200+mph Mercedes AMG models were actually "tested" by the people of several automobile magazines.
In many countries, Mercedes-AMG does offer these 200+mph to their customers. Mercedes-AMG knows that their customers won't be happy when they a Porsche pulls ahead of them after 250kph or 155mph.
But eventually the cars are still limited at about 205mph, because no tires are capable of that speed when the car weighs around 4200lbs, until Michelin comes out with the tires for the 4100lbs Bugatti Veyron.
Bugatti which is also VW-owned, had claimed a 252mph top speed for the Veyron, but this is nothing but a claim. Michelin is still trying to develop a tire that could actually hold up to 252mph. And also the body work of Veyron is still not done. Same for the transmission, the transmission just couldn't handle the 900+lb-ft torque.
If anyone know any real test (other than VW's) please let me know, because I don't really believe what VW says.