I was running mine silly low (-100mm) on 16's. Purely because it looked the dogs danglies, however the flip side was scrubbing on full lock even though I've pulled the arches (need spacers). Used to catch the pavement when parking, beached it outside the hospital on their speedbumps, generally nasty to drive.
Raised it today about 80mm as part of "winter mode", I can now actually drive the car without worrying about how low it is.
To the OP general rule of thumb is so the wishbones are level with the ground, anything else is will compromise handling. That would be approx -60mm from stock ride height (-40mm on the 16v) Combine that with a decent set of tyres on 15's and it'll look good and go round corners. Try to get the lightest wheels you can too as unsprung weight makes a massive difference too.
