Is understeer harder to regain control with wider tires?
 

Is this true? Say you run a 215 to a 245, if you happened across understeer, would the 215 be easier to gain back control of the car?

This is not something I have personal experience with but I would have thought the break away and then gripping again characteristics would be more to do with the tyre compound than the width.

what kind of understeer are we talking, little scrub or full on audi plowing.

I don't think width really makes a difference but more so the tire compound and also how communicative is that tire. I ran NT05's on my M3 and found them to a fast tire but not the greatest in terms of feed back. Now that car is on michelin super sports, maybe not as much overall grip but the better feedback really helped me know what was going on.

Wider tires will exaggerate the understeer, yes.

+1 for Audi-plowing. I miss the torque of my B6 S4 with it's v8 hung out past the headlights, but man, the steering wheel was really more of a "suggestion device".

I got a laugh out of audi plowing. My B5 feels like a boat when I go back and drive it.

I imagine the reasons are two fold: (if this is incorrect, I apologize).
The more available grip you have means you would be at a higher limit when the audi plow begins. So I imaging it could be more difficult to regain all that lost grip since the forces causing it are likely higher, even more so as the tire overheats from scrubbing.

Also, a narrow tire can respond a bit more quickly, making it easier to detect and correct understeer.

I always thought wider tires just allowed a tire to wear slower. @CSG Mike?

THey allow a tire to heat up slower from the increased mass, but it can add grip too if the contact patch is increased.

As the surface area increases (assuming weight doesn't change and while a larger tire will weight more the difference to the totally weight per corner is negligible) the pressure decreases, meaning there is less force on each unit of the contact patch. There are more units of surface area with a larger contact patch though, so the reduced force is canceled out by the increased contact patch. Thus leaving the force due to friction as simply the product of the coefficient of friction and the normal force.

Am I going wrong somewhere?

The pressure decreases, but the total force necessary to break traction increases, so you can still corner harder. Harder cornering increases the pressure on that corner....