I haven't ever driven a SHO, sans 20 inchers. I have, however, driven a new SHO with PP and one without, but BOTH with 20's.
I did notice a considerable difference in ride quality between the two. The PP equipped car is certainly not bad, in regards to NVH. Just not nearly as supple.
I agree that I believe there is still enough sidewall on the 20" tire, so that ride quality isn't hugely compromised.
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