I'm building up my engine next spring but, wanted to get a few basic parts now. One of which was an Apexi N-1 catback. My car will be turboed next spring so I figured on getting the 3 inch piping. How will that affect my n\a performance? Should I get the gst option? Will I need a flange or couplink to link the 3 inch piping to smaller stock pipe. (I own a 99gs)