Do taller/wider tires work better than the stock sizes?

The stock tire sizes are pretty good. It's getting harder to find 100/80 and 130/80 tires, though, so it's good to know what the other options are. Many people write in and want to know what the widest tire they can put on the 250 is, but they really are misinformed. A too-wide tire can kill all that the 250 has going for it.

Bigger tires cause more effort needed to turn, use more energy to accelerate (and decelerate), plus the added weight will provide more gyroscopic effect, further hurting handling.

You want the SMALLEST tire possible without overwhelming it with power. On the 250, that's about a 110 (not really available) or 120/80-16. Wider does NOT give "more" cornering clearance, ability or leeway.

Taller tires alter your steering geometry; this can be good or bad, depending on which tire you change. A taller rear tire will steepen (decrease) the geometry, making the bike turn faster; a taller front will increase the steering angle. Increasing both front and rear tire heights can be good, because you get an even steering angle (close to stock), and get added road clearance and trail length (provides stability). A taller rear tire will also increase the final drive gearing a bit (lowering cruising revs), while a taller front will increase speedometer accuracy.

Wider tires generally slow the steering of the bike down. They also alter the contact patch, as the tire is 'pinched' in the rim. Wider tires can provide better straight (non-leaning) traction. Wider tires are necessary when carrying heavy loads, or if the bike makes a lot of power; the EX250 typically does neither of these things, so wider tires are usually of no benefit.

Taller and wider tires both weigh more, so you have additional unsprung weight to deal with which slows down suspension action.

Note: The 130/90-16 (rear) and 100/90-16 (front) are the most popular sizes for EX250 owners when they replace the stock tires, if only for the fact that there are few other choices. 80 series tires currently available are Pirelli MT75 and Avon AM63 Viper Stryke.

The difference in ride height between 80 series (MT75) and 90 series tires (BT45F/ME880R) is a little more than 1" (1.5" in the rear).