I ran a couple Mach 1's when I had my GTO. The first one was modded to the extent of a tune, an intake and a shifter. I won the first race, he said he missed a shift. We hot lapped and he beat me by a tenth. I cooled off real good and ran him again and beat him by about 3 tenths. The last run was about the same as the 3rd.
I ran a bone stock one and beat it by over a second, but the driver really didn't impress me.
Most of the stock Mach's I've seen are mid to high 13's. They are about 3-5 tenths slower than the GTO.
I'd agree with you that Ford got the Mach right. I would still take a Cobra over a Mach. The Cobra is about 4-6k more money than the Mach but for that you get nicer seats and gauges, the supercharger and stronger block and internals (worth 6k+ right there) and the IRS. Cobras handle and ride a lot better than the Mach. If you can swing the extra$$$$ go for the Cobra although you won't go to far wrong with either.
Better yet for the money you would spend on a Mach you can get an 04 GTO that runs with it and rides and handles better. The GTO gets better mileage, has a nicer interior and a much more solid car. Or for Cobra money you can get an 05 GTO that is slightly faster, gets better mileage, rides better, has the better interior and is more solid. Again you won't go wrong with any of these cars.