At that elevation you llikely won't see near the gains others have reported, but you should be fine on 91. As for you question on octane boost in a bottle...Don't even bother. Too many tests have proven them ineffective at actually raising octane. The only exception I'm aware of is a Torco additive. I do know some who've had good results using that on other types of boosted setups. The other method is find someone who sells unleaded race fuel and mix it with your pump gas to the desired ratio.