I disagree that you HAVE to go full system to get gains. In FACT, manufacturers put more effort into developing headpipes than they do endcans, since they KNOW many ppl change them anyway and you can only do SO MUCH and still pass strict emissions standards.
Hindle rep explained that they aim for peak gains of 3-5 hp from an endcan and 7-10 from a full system on today's bikes. And he also stated that they WILL NOT release any exhaust parts unless they test them to improve on the bike's power delivery THROUGHOUT the power band.
Do you HAVE to rejet? Some manufacturers say not, but because you will no doubt be increasing the airflow from the engine, it only makes sense to adjust things on the input end of things to maximize and optimize the gains that can be had. I have HEARD that fuel injected bikes are better able to compensate for this by adjusting the mixture, but I have also heard that this is just a MYTH, and fuel remapping has to be done again to optimize gains.
"Keep yer feet on the pegs and your right hand cranked."