Not had a proper look yet, thinking 12mm or 15mm.
that's a fair bit. not sure what the limit would be, before you'd have tyre-rub. i would think 10mm is as much as you'd want to go; that's the equivalent of an offset of about +28mm on a 7.5" wheel (presuming the wheels you have are 18x7.5" with +38mm offset).
might be ok on the 'straight ahead', but full-lock on steering will probably rub. may not be a big deal.
in terms of handling, it will depend upon the wheel alignment. to facilitate turn-in, you might need more toe-out, but that won't help tyre-wear, and may not help straight-ahead stability. neutral toe should mean the car won't turn-in quite as sharply (bad). the difference becomes more apparent with a wider 'track'.
so, with normal track (ie: normal width, no spacers), toe between +2mm (slight toe-out) and 0mm (neutral) might be about right. with a wider track, you'd probably need toe-out of +5mm to achieve similar handling to +2mm on the normal car.
significant? probably not.