to help you out, why not buy an x-pipe that has primary delete but retains secondary and resonators? you'll easily spend 1,100 plus 399 plus labor and at the end of the day the gains will be the same, if you just buy test pipe.
20hp max from x-pipe and 18tq and with a tune another 20hp or so depending on the tune and intake will never give you 15hp thats never the case, more like 8hp.
search for threads my dude
48hp or so with everything. if you want the real deal buy your self a blower (ess, active)