brake disc machining (slotted/groved)

at a trackday over the weekend i was talking to a lad with a 197 who had his OEM discs machined to have slots or grooves in them

I've got a new set of brembo discs ready to go on my car in the spring - i fancy the grooves though for better pad cleaning and they look pretty good too

anyone know if its ok to machine the OEM discs and anywhere good which could do this job?

thanks