James, you can fit a 295/50 (~10" tread width and ~12" section width) on the standard 9.5" rim and with the factory Trigo adapter spacer in place. However, the edge of the tires will come right up to the fender lip and this limits your ability to drop the ride height. Mod #1 is worth the hassle of having the inner hubs machined and shortening the pin studs for this reason alone.
15" vs 17" wheels really don't change the maximum width you can handle, but altering the backspacing is the key. Also, the tires that are sized for 17" wheels all tend to have a lower aspect ratio (i.e. 40 or 35 series, vs the typical 60 and 50 series you see for 15" tires). The lower aspect ratios produce a superior handling tire (due to the shorter sidewall), but some Cobra afficianados will argue that the larger rim/smaller tire combo don't (a) look right and/or (b) fill up the wheel well properly. I've looked at enough Cobras to appreciate both, so it comes down to personal choice.
If I ever track my car, you can bet the farm I will be running 17" wheels and tires. Most of my driving today is just bombing around town, stoplight action and the occasional canyon run. That's why I went with this set-up...to maximize my traction in those instances and to not get unnerved when I lose traction on the 1-2 and 2-3 shifts. Some here like that feeling...I just don't trust my driving instincts with a high-powered, short wheelbase car to correct the slippin' and slidin', especially on the street with curbs, light poles, pedestrians, etc.
-Dean