Will these modifications give me a significant power boost over the stock set-up and are they worth doing?
Yes, not to mention cooler running and longer engine life.
Look at the 250X forum under the 'stickies' at the top of the page and you'll find all you want.
There's a lot you can do that can only be done
simultaneously, such as jetting and airbox modifications. That is always where to start.
Most everyone on 250X's uses the excellent JD jetting kit and follows the directions, included also with the kit are directions and pictures of the airbox modification.
Honda molded lines into the plastic to show you where to cut, but again,
simultaneously with jetting
only.
Also very nice to have is the crfsonly/Kouba fuel screw.
The exhaust can stay as is, you can cut the discs off the back of the insert and dril a big hole in the difffuser, some just remove the diffuser (to loud for me ), some instal a slip-on or a whole 2004-2005 250R exhaust.
Again, no exhaust mods without jetting/airbox first or simultaneously. Doug