Which one is the downpipe? And which one is the catback? If the downpipe is the HKS one, then most people would advise you to switch out to another downpipe. If the catback is a HKS one, then you will love the sound. They lean toward the louder side of the spectrum. What kind of Megan/HKS did you get? HKS Hi-Power? Carbon Ti? Megan?
haha actually that's a good question because i have the megan downpipe and i'm planning on getting the HKS Carbon, so i believe from my research they fit. Like Tuner Ready said if its the HKS dp i'd switch that out for something bellmouth
I have the HKS Carbon-Ti cat back on my WRX and I can tell you from personal experience that it mates nicely with both the stock dp and Cobb's catted dp. I don't think you'll have a problem with Megan's dp because it's a pretty standard size.