I agree with Pete.
Personally I start hunting for tyres once the tread drops below 4mm, largely 'cos my previous car (Merc SLK 350) was like a drunken moose on a skating rink once the rears dropped below 3mm and the weather was wet/icy. And that was WITHOUT braking
Fronts didn't seem to be affected until nearer 2mm though.
Have to wait & see how the 320 fares when the time comes, but I doubt I'll be letting them go much beyond 3mm in any case as I've seen too many victims of RTAs in my time - mostly arising from an "inability" to stop smoothly and appropriately.