I recommend smaller as possible. I had 5mm on mine and didn't feel much difference or issues but before you do that google "scrub radius". You are altering the suspension geometry. The geometry is set so that the tire rotation happens at center or near by pushing it out, the tire is rotating 10-20 mm on the inside. Just google it. 20 sounds crazy to me from a performance point of view. I wouldn't go more than 10 or 11. Good luck.
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