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Depends what aspect you're talking about. Conventionally, a shorter sidewall will allow less flex which in turn reduces traction. However the stock runflats are pretty stiffly sidewalled anyhow, so it might not be that bad a hit. Along the same lines, the reduced flex means better cornering capabilites as well....so straight line traction may suffer, but your cornering grip should improve.
Here's some pics of my Z...the rest can be found in my imagestation album (there's a link in my sig). I've since lowered the front of the Z another inch or so. Much better :thumbs:
Weight of the rim has some performance factor also. A set of lightweight forged 20" rims will offer maybe a tenth or two of benefit due to unsprung weight being 4x+ that of non-unsprung weight.