I just bought a 2006 WR250 a couple of months ago. Yes, removing the AIS will improve performance and no, the stock needle is not adjustable.
In my opinion the bike was barley ridable as delivered. It had very little power and stumbled badly when accelerating. It certainly did not compare to my 2001 YZ250. But it came with a green sticker which was why I bought it.
I wanted to get as close to the performance of my YZ as I could with as few changes as possible. The bike was still gutless after changing the slow and main jets so I removed the AIS. That made a huge difference but it was still lean in the midrange so I installed a 2006 YZ needle. I also changed the pipe and cut the thottle stop screw. Now the power is very close to my YZ.