Why keep the plate though? Surely if you're changing car to a normal 5 series and have a plate saying M5 you're just going to look like a ****er wishing they had an M5 to most people.
Why change it? It's really hard these days to buy a new single number plate anyway as most of them are gone, he has his initials on it and 99% of people wont link M5 and a particular model of BMW.
My plate starts S2 and I don't drive an Audi S2. OH NO!
Why would you want to be out pottering along on a bit of NSL on a saturday, surely if you're into the whole performance car thing you're into speed? Not doing 5-10mph under the limit like some old woman?
What if he wasn't in the mood?
Sometimes I like to potter along in comfort. Sometimes I like to wring every last bhp out of my engine. Depends on my mood.