Who ever said anything about stock internals?
If you want an extremely fast, streetable car, I'd take a 2.6 with a big turbo over a supercharged v8 anyday.
No one is arguing ultimate potential, but if you want to argue bang for the buck.
For the thirty plus stacks your gonna spend on a cobra, you could build a s13 with an awd rb26dett with internals, a strong, capable drivetrain from clutch to tires, and prep the whole body (cage and everything)
At that point youd probably have a 240 with an engine capable of supporting big big big power, a chasis with the ability to put it down, and from a baseline its already lighter and faster then the mustang
then start spending loot on the mustang and the 26 is gonna be looking at bigger turbos fuel upgrades, blah blah blah.....oh whats that, two ehads to work on, two cylinder banks to buy parts for, the 26 only has one......