There was a dyno posted on this forum in the past year where the guy had a stock pull at around 17hp and a stage 2 pull with the usual aftermarket goodies and it went 23hp on the same dyno. That folks is a difference you can feel. I'm not sure my numbers jumped quite that much but I can say that my bike sure liked being opened up and jetted. I would have a dyno pull to prove it but the local dyno won't do a pull as long as I'm running knobbies.

 

The stock muffler uses a bunch of passage ways, drilling out the outlet will do absolutely nothing in terms of opening it up. Besides, it really isn't the muffler restricting it, it is the catalytic converter.

What FMF did you buy? The Q4 is the quietest aftermarket muffler you can buy. But the Q4 is definitely not that loud. I am thinking you must have gotten the powercore 4 instead? A header pipe with an expansion chamber with knock a few more decimals off as well.

Yes, you can just buy the jets you want, however doing it that way doesn't come with any instructions and you need to know what jets you need. Plus the dyno jet needle is worth the price of the kit.

Maybe you didn't get yours jetted quite right ? Did you open up the air box at all ? It made a big difference on my bike. Like Linkin posted, there are several dyno's floating around that put the power from 18 hp to 22-23 hp when opened up with exhaust-intake-jet kit. That is over a 20% increase in power, which is what it felt like on my bike.

It was definitely worth it

 

Sorry if I shot off my (keyboard) mouth without really doing it myself. I just figured the main restriction on the silencer was the outlet. It's tiny.

As for the header. It's a fairly thin wall pipe. I don't see that as a restriction on a 250.

And for the needle, again, I don't have any data to back it up. With everything else staying the same, I just didn't think the needle would make a perceptible difference.

I got the jet kit and the Q4, UNI filter and opened up the airbox. Overall, I'm happy with it overall even though it's loud for some reason. Louder than my KTM 450.

After all the mods, the bike can hit 90mph. Pretty impressive. Maybe one day I'll prove it with a GPS and a video. I'm quite happy with the acceleration and top speed of the little bike. She's certainly a keeper.

 

I found this link long ago from another member's post (I believe it was DirtSurfer' Mod Day, in the stickys)
Kawasaki KLX 250 slip on - HMF Engineering Forums

Two good things about it: first, it's a "how to" on carb removal and rejet for the KLX. Second, it has a dyno chart, stock (18.3 hp) and re-jet/pipe (20.8 hp). They shimmed the stock needle (I believe there's something to the DJ needle which may have yielded more power). They also removed the snorkel.