I think about the only thing you have on your side right now is science. You'd be hard pressed to find someone do a real-world side-by-side comparison testing only tire widths(I'd assume different wheel widths as well).
From a simple physics standpoint, one would argue no difference.
f = u*N
or friction equals the coefficient of friction times the force pressing down on the tire
Of course it's not this simple. For one, we have heat. A wider tire has a greater capability to disipate heat and stay cool. As well, the wider tire also has less force per square inch applied to it. A 265 width tire has only 75% of the load carrying duties as a 205 width tire. Less work, less heat, less wear.
The larger contact patch also provides more area to generate traction, larger area of contact with the road and more biting edges.
As well, the coefficient of friction isn't linear as more force is applied. As more force applied, there is less and less of a gain. You actually gain a greater benifit with the less force per square inch.
It's much like making a car faster by lightening the car or lower the CG of the car. Anything you can do to minimize the work the tire needs to do helps. A wider tire just spreads out the force more and makes the work of the tire that much easier.
All cars would have super wide tires if there were no negatives to using a wide tire. Despite the benifits, there are problems as well.
One obvious one is camber. When you make a tire wider, it becomes more camber sensitive. This is kind of dependent on the wheel width and aspect ratio too. You can run a 235/70/R14 on a 6" rim and it may not care so much about camber due to all the flex available. You definately wouldn't like the handling, but camber wouldn't be a problem. A more appropriate setup would be 235/40/R19 on a 8" rim (just picking random numbers, nothing specific to our cars). This would handle a lot better and is sized more appropriately. However, the low flex available would make the contact patch very sensitive to camber changes.
You also have the physical limitations of the car. You can only fit so wide of a tire in the wheel wells before you start rubbing on things, either fenders or the struts/coilovers. With the front tires, you may even rub in the wheel wells themselves during a turn.
There's comfort as well. Handling was kind of metioned above. You can range from a narrow high aspect ratio setup to a wide, low aspect ratio setup and vary the handling and ride qualities of the tire. On a narrow rim, you'll produce an increasingly sloppy handling feel as you widen the tire. So there's limitations by the rim as well, both physical and desired handling.
A big issue with wide tires is the idea of deformable surfaces. A wider tire isn't always benifitical on all surfaces. On a solid surface like asphalt, yes, wider is better. However, on a deformable surface like gravel, dirt, or snow narrower is actually better. Since these surfaces are deformable, i.e. they change shape due to pressure. We can use a narrower tire and actually compress and dig into the surface gaining large amounts of grip, at times more so than a tire can generate even on asphalt. Think ruts and grooves here. A narrower tire even helps on wet surfaces. The higher contact patch pressure helps force more water out from under the tire...rather than floating over it.
Then there's gas milage and friction. A wider tire has more grip and generates more rolling friction. As well, its wider swath also has to be pushed through the air, resulting in more air friction as well. Milage drops can be quite noticable. This is also dependent on the particular tire, it's rubber compound, and even its air pressure.
In the end, how wide a tire is best depends on what surface(s) you plan on running on. Where do you need traction the most? What trade-offs are match your needs best?
It's not as simple as wider is better. There is a best match for you. It may not be the same match as another person. Wider is generally better, but how much traction in other areas are you willing to give up?
This isn't much of a quantitative response. But, it's tough to give exact answers as there are many different results depending on many factors, things like car used, suspension setup, outside temperature, surface conditions(cleanliness/type), even the tester's driving behavior will all affect the results.