im goin to haVE the local meinike redo my exhaust system if i provide them with the specs of a cat bac like a "Megan racing" or a "bosal" cat bac im i still goin to get the power gain that a cat bac provides?

Anything that is better than stock will free up some power. If possible, find a local shop that specializes in performance exhaust, they can probably make you something better than Meinike can for about the same price.