Dutch, i have to disagree on one part here... my bike hardly run with Leo slip-ons and stock mapping... kept stalling every time RPM dropped, and generally run like absolute shit. maybe if you have restrictors and baffles in the slipons, it would be better, but with 'open' leo's it run dangerously lean... i don't know if Euro/NA gas difference plays any role in this... and few people report running stock map with slipon and even with MH airbox without problems, so it's hard to say... this was my experience.
i would strongly advise getting PC/tuneboy and having custom map done... best-spent $250 ever! (well, 250 for the map, another 250 for the PC... still worth it!)