Man, if you want high tech and state of the art, the first mod you should try is "Optimum-Power.com's" top of the line programable, (on the fly "map" switching) EFI system. Currently they make the system for all open class YZ-F's from 99' to present. With the system you can either upload their pre existing "maps" or make them yourself. 1. Change "maps" on the fly with the press of a button. 3 maps can be saved in the brain a the same time. Going up a sandy hill? Flip the button and change to a map that favors or enhances low end and mid range. Imagine the look on your competitors (or buddy) face as your bike comes on the pipe. No shifting or clutching on his part will match your acceleration. 2. Hitting a long straight away? Change maps to favor top end speed/ power. Watch the look of puzzlement on your partner's face when you pick the front end up on a the straight and take off like you hit the NOS. 3. Way out in the desert or woods? Running low on fuel and change to an economy type map. I don't believe it add's a ton of HP. But it made my bike night and day. I would think only cams, valves and other engine mods could add significant HP. It's more about throttle response, and power band enhancment as well as emissions. A company called REV Motorcycles currently makes a totally customizable, green sticker 2 stroke 250 with electric start and EFI that only weighs 214lbs. Anyway, with Optimum's system it only adds a few pounds and utilizes much of the stock system. I guess they also make the system for 2 strokes. They key is being able to do simulations and be able to see what will happen without buying and trying new parts and mods or doing it trial and error style. Toyota, S&S (huge Harley aftermarket company), Roush-Yates (Ford racing), Cannondale MX motorcycles, and RAETech use their control and software systems among other things. Their software itself is state of the art. The software can do parametric design, optimization, Virtual DAQ, catalyst and MATLAB/Simulink. Includes turbocharger, supercharger and intercooler support; Co-simulation option with FLUENT; Transient pipe temperature analysis; Variable pipe length; Multi-user database and Dynamic valve motion. Obviously you can use the system for many applications besides on the Yamaha. They start at $2,500 and go to $3k for the top of the line. I fiddle with mine on my laptop IN THE DESERT and then upload the changes on the spot. I can then switch from the 3 "maps" while riding. This is a a fairly inexpensive mod for serious racers or off roaders since you won't need many of the other mods you get in it's place. In that sense you are spending the same as if you'd modded the rest of the bike. What I did was to buy a beefed up and pro-modified 99' YZ400F for $3,200 and then put this on. Much of this was because I needed a green sticker YZ-F here in SoCal. I am already making the same HP as a slightly modified 2005 YZ450F, although obviously my bike is heavier. My entire suspension was already re-done. I'm still out less than a new YZ450F and smoke every one of them I come accross. Keep in mind I'm not MXing for a living. This is not the future, it's today. Not having programmable EFI is like riding a 78' XR80 in the Super cross. This is some trick stuff people! Hope it helps!