The UK's Bike magazine (my personal favorite for sportbikes) had a can test last month. No mid pipes mentioned just bolting on slip on cans, approx 8 different cans. Scorpion did very well. Most cans 5 horsepower gain with no rejetting, some cans hurt fuel air and required work. ( Yoshi did ok).
Companies like Scorpion claim their whole idea is to make a nice can for you to use without rejetting. They are spending big R&D dollars (according to them) on building cans which do not negatively effect our carberation.
My D&D that comes with a mid pipe for 600S requires no rejet according to the facotry (I called them and called the rep, couldnt beleive it) and it has no adverse effect, only nice punch in the mid range and wonderful (LOUD) sounds.
I beleive you will see after market exhaust manufacturers striving to make cans we can use without further mods. Its what the majority of riders (who want a small gain for a little work) want and money and numbers talk.