Speed World Challenge has no rule disallowing AWD, R33BusaMan. However, only the Audi RS6 has been able to employ AWD with any success on track.
The JGTC is hardly a gauge of production-based sports car racing. Serious road racing fans refer to it as "Japan's NASCAR" for good reason. It is highly unlikely that this Skyline GTR will "kill" the factory Autohaus Viper driven by Tommy Archer (you know, 2 time SWC GT champion and American Viper expert Tommy Archer).
I do think that the Woodhouse and McCann Viper teams will have a look back over their shoulders at this car, as will Stu Hayner's GTO and the new Aston Martin DBSR9's. However, factory teams like Autohaus or the Pratt&Miller Caddys are a whole different animal.