With a cdi on your 650 rejetting is not absolutely necessary. You can leave the bike with the factory jetting and it'll run fine, sure you may see 1-1 1/2 more hp if you rejet but it's not a must. With an exhaust on the other hand rejetting IS a must. These motors can be a little touchy when it comes to air flow. If you put on a pipe and don't rejet you'll actually lose power and run hot from being to lean. Basically you can plug in the cdi and you'll be fine, sure rejetting would help but it's not a must. If you go with an exhaust you will have to rejet also, if you don't you'll lose power and could hurt your motor. That's why IMO the cdi is the first mod that should be done. For many people it's the only performance mod they'll ever want or need.