This is a complicated issue. The RULE of thumb is start at the head and work down. But, Honda made a great header with proper dimensions and 4-2-1, so it is really hard to improve on it. Changing the cat to a testpipe gives a couple hp, but not really enough to make it worth touching given the EPA rules. Basically think Tune, CAI, matched header, resonated TP, 70 mm single exhaust and you will have a real return, everything else is really pointless.