K-series motors have the infamous "cat in the header". Some people use shorty headers to keep their catylitic converter, but only gain a few horsepower. The best bet is to go with a race header, like the one in the store, which kind of functions as a header and downpipe in one. Although there are trade-offs. You won't pass inspection (I would just put the cat back on before inspection) and if Hondata makes a reflash for our cars (which they will) it isn't recommended to run the race header with the reflash as it causes the car to run dangerously lean. K-pro, which is tuneable, is fine though.