Hey XR100- The needle doesn't affect your bike's performance at full throttle; your MAIN JET is your key to full throttle performance (refer to your manual). Since you mentioned that yer KX sputters (usually means rich) in the power band (close to or at full throttle), my guess is that you really need to change your main jet.
I ride a KX100 too and typically change the MAIN JET every time I change from a lower altitude (bigger jet) to higher altitude (smaller jet), or every time the weather gets significantly colder (bigger jet) or warmer (smaller jet). It's EASY and takes only five minutes tops.
Here's what you need: I got a small jet tool for under $5 at the bike shop to so that I don't even have to move the carb at all. That tool is simply a short stumpy-looking 6mm hex socket. Nuthin' special. You'll also need a 17mm closed or box end wrench and a 10 mm box end wrench. You'll also want a couple of extra main jets from the bike shop. The stock main jet is 138 typically for sea level. I carry that, plus a couple larger (for my aftermarket pipe) and a few smaller (for mountain riding).
Here's how: Turn your gas off. Take your 17mm wrench and unscrew and remove that thingy (technical term) on the bottom of your carb. Some gas will leak out of your carb for a minute or so. Stick that 6mm hex socket up into the bottom of your carb and unscrew the jet. The long-ish brass thingy that comes loose is your main jet and needle jet. Take that long thingy completely out into your hand. Now, put the 6mm socket over the main jet and use the little 10mm wrench separate the main jet from the needle jet. Replace the main jet with another one (I'm guessing smaller size to fix your problem). Then do the steps backwards. One note, don't tighten the brass peices too tight (just nice and snug) since they are very soft metal. Try to keep the parts dirt free (could cause carb clogging later). Remember to turn your gas back on (I forget all the time).
To check to see if the jetting is correct: Put in a new spark plug (I do use one that's a little hotter than stock). Ride around like a banshee for five or ten minutes. Stop the engine quickly after reving it up a bit more. Remove the spark plug and take a look. If the end is black and oily wet, this is too rich so try a smaller main jet. If it's white and flaky, this is BAD and too lean so try a larger main jet. If it's a nice brown carmel color and dry, they you GOT it! Note, it's better for your engine to run a little rich than to run lean. You could seize your engine by running it too lean.
I'll add this too: I started with a 32:1 pre-mix at first. Since I ride at altitude and my bike's broken in, I changed to a 36:1 or 40:1. It may not be as good for the engine, but I get better performance and less exhaust spooge. It's probably not the smartest thing to do.
I REALLY found that changing my main jet is THE KEY to keeping my KX100 running like a TOP! I also try to keep my (aftermarket for better airflow) air filter clean, change the gear oil often, and repack the pipe more often than you'd think (because it gets real spoogy in there and constricts airflow).
Sorry for the length of this.