My 400ex has all of the usual stuff, White Bros E series, K&N filter with outerwhares, jetting and no lid. With this in mind, would I benefit more from a CDI(little black box) or a sparks timing key? Or maby both?

 

the box won't do much if any unless you have a built motor where you can turn the extra rpm's. the key may give the bike a little better throttle response. the key may also make the bike run a little hotter. for the price, the key is probally worth it, but you won't notice much besides crisp throttle response.