Exhaust wave timing. That subject could get a 100 page thread started
The placement of the X - H is very important. X and H pipes operate differently and thus have their own strengths. The bends in the pipe are important. The type and sizes of the muffler are important. If the company did research their will work much better than a company that just throws in stock parts at it and makes a new system just so they offer one for that model car. So how do you know which one works for your setup? Well the key is research. Find others who have true before and after dyno's with your exact setup, find manufacturers R&D dyno's. If you start looking through dyno's comparing systems, look for power dips, etc. Think about the rpm range you are trying to maximize and pick the one that makes the best power over that range. If the company doesn't publish their R&D dyno results, then I figure they didn't do them and were all about designing a cool sounding system over one that actually performs well.