If they are just slip on then you shouldn't need a gasket...the only gaskets are where the header pipes mount to the engine. They should just clamp on. I put baron customs slip ons on mine last year. They just had a band clamp around them
A little, enough to notice a difference. I noticed it more with running non-ethanol fuel. Biggest improvement was after the XXX LA choppers open air filter kit. The cobra slip on was a definite improvement.