91 octane would actually make it harder to pass smog because the higher the octane the harder it it to ignite and burn completely. 87 burns very quickly and ignites easily, witch is why it is horrable to run under boost or very advanced conditions. But for a smog test 87 is great because you never see boost on a smog check and you want your timing set to spec, not advanced, so you can run a lower octane fuel to keep hydrocarbons counts down and have a more complete combustion.
Having said that. I would like to add that If your vehicle is kept up with regular maitnance and is in good operating condition with your emissions equipment in working order then you should have no trouble passing on 91 or higher octane.
I personally have to run 91 with octane booster because ca gas is sh!t
sorry for the hijack