I think the SS also come with a different exhaust system. Other than that, from the track results that I have seen the Z and SS are very close to each other performance wise.
SS owners just like to say that they have an "SS" whoop-D-Do-Freaking-Good...you paid how much more to get some dinky SS badges, a slightly different hood and different exhaust tips? who cares.
Ditto on Mach 1's statements.
E
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1991 5.0 LX Coupe - 38,000 miles
13.17 @ 106.14 mph w/ 2.138 60'