I've tried all four combinations of stock pipes and straight through slipons with stock airbox and jetting and open airbox with modified jetting. The pipes make almost no difference in power except right at the top of the rpm range. The open airbox and jetting makes a big difference that you can easily feel.
I was very happy with the results of my Stage II open K&N and pipes. Opening the airbox is awesome and lets out a great intake roar. My results were very good following the dynojet directions. I dyno'd my bike directly afterwards and the air fuel mixture was perfect.
These mods were on a 750, don't know how close this is to your 600 but thats all I got for you.