My dealer admitted to filling mine up with regular unleaded (85 octane here in Utah) when I picked it up brand new, I noticed after I ran that fuel out and refueled it that the second tank got a little better MPG and ran just a tad snappier but even with 85 octane it ran OK and didn't smoke at all.
I have heard cases on others SS's of bare wires grounding or shorting out in the engine compartment, check all the wire harnesses on the driver side of the engine that come from under the painted silver plastic spark plug cover or better yet tell the dealer to check them or call GM Tech Assist for help if they can't find the problem.