The accelerator pump modification is a great/cheap upgrade and will definitely improve your skills. It basically "syncs" (for lack of a better term) your carb so that when you get on the gas it doesn't hesitate/bog. The 4-strokers power band is great in mid range but when you're riding technical trails at lower speeds they all want to hesitate when you really need that front wheel in the air to get over an obstacle. It improved my riding skills in that since that I was finally able to wheelie over things rather than bash it with my front tire and deal with the repercussions. Just remember to change out the o-rings every season and carry a couple in your riding bag. They seem to dry out/break easily.
Another change I made this last season is switching to a trials tire in the rear. I debated this swap in my head for years and really wish I would have changed years ago. The traction is insane and they last 4-6 times longer than a traditional knobbie. My only complaint is that they look pretty gay and if you make the swap you've really got to consider this http://tubliss.com/