I don't know anyone riding a GasGas250 or 300 with a #6 slide. From what I read here, the EC200 sometimes is stock with a #6 slide.
I rode a little over 40 dirt miles with the new needle. I started in the leanest clip position. My bike ran great, but the bike reved past the sweet spot of the power a little too quickly. I dropped the clip/raised the needle and slowed the revs down a little and rode most of the day like this. The power is linear, but it rips onto the powerband with just a flick of the wrist. The power is very '99 YZ400 like, but with more low-end. Mikey Likes!!!! Now, I just need to test the next richer needle position and also with my #8 slide.
You may missing some performance that you don't know that exist with your current jetting. However, if your happy with what you have, then good for you. There are no "set in stone" jetting specs for our bikes. If you search the site, you will see the bulk of riders here using a 40 or 42 pilot jet. It also appears the bulk of the riders using a #7 slide. Needles? do a search... Main jets vary the most, and I seem to run leaner than most with a 172 at sea level.
Mike Baxter