I doubt 12-15 HP per se but an X pipe works wonders accoustically for noise reduction which then seperately but related allows you to run a less restricted muff/pipe dia rearward from the collector flange which WILL gain you HP as any restriction down stream from the header collector flange (collector reducer/smaller pipe dia/quiet muffs) costs power & you're limited by amt of noise you (or the local constabulatory) are willing to tolerate. backpressure/pulse wave tuning is a primary tube issue only & is very critical to achieve an optimized combo EDIT pri tube length/dia