About wider tires
Lots of new people around here ask about putting those cool, phat tires on their 250 that they see on bigger bikes and on TV. This is a really bad idea, and if you can't live with what we just said you'll be better off getting the 600 supersport that you really want.
The EX250 rear rim ('86-07) is only 2.5" wide. Anything wider than 130mm gets pinched in the rim and really compromises the handling that people think they're going to get with a wider tire.
While the EX250J has slightly wider wheels, the same guidelines apply to mounting tires that are appreciably wider than the stock sizes.
For more information on this subject:
Do taller/wider tires work better than the stock sizes?