Well, from the bare facts given if the M5 is electronically limited to 155mph and the vauxhall ISN'T limited and can do 160mph - then clearly the Vauxhall VXR is the faster car, at least as far as top speed is concerned (unless the BMW is limited to 155 but can go faster when you ask it nicely).
Or is there a conversion factor from BMW mph to Vauxhall mph that I am not aware of that changes the above figures?? (eg 1.0 BMWmph = 1.2 Vauxhall mph)
- or am I just thinking like an engineer?