Save your money and get a full system and a programer at the same time. This way you can tune it all at once. Realistically a slip on will only give you a bit of power by itself and any good slip on should be programed. Many guys have gotten away with just adding the slip on but these bikes already run on the lean side from the factory. It really depends on your elevation as well.
2008 700XX - OMF Beadlocks w/Billet Centers & Reinforcement Rings, Kenda Kutters, CCP Steering Stabilizer, ASV F3 Levers, ODI Rouge lock-on grips, Houser Bounce Nerf Bars, Pro Armor skid pad, UNI Filter
2007 400EX - CCP Steering Stabilizer, GT Thunder 450R Shocks, 450R Rear Brake Conversion, YFZ450 Front Brake Conversion, ProTaper Bars, Rath Racing Nerf Bars, Alba Elite Bumper, ASV F3 Levers, ODI Rouge lock-on grips, UNI Filter, Kenda Kutters, Pro Armor skid pad, Armadillo Swing Arm skid pad