Can these results be indicative of how an engine would act under boost as well? Dyno testes with all these items would be a true test of benefit of each. Too many variables left just checking individual tubes for flow.
These results are just one test and this isn't the definitive answer to what works and what doesn't.
The way that Ford chose to smash all three tubes into one 2.5" collector simply doesn't leave enough room there for enough cross sectional area for 3 tubes. It's just not there. Without quite a bit of welding (which is going to cause extreme distortion) you just can't get the proper cross section.
Clearly Ford realized that the 95 header is a bad design because they changed it on the trucks, Tbirds, and 99+ mustangs. They basically went back to a tubular version of the old cast iron manifolds.
While many people like to think that the larger 2.5" collector is better than stock they fail to realize that 2.5" is too big for the size of the tubes dumping into it anyway. 2 3/8" has been calculated to be the ideal collector for 1 5/8 tubing (on a real header). Since the Mustang header is 1 1/2" tubing, it remains that the collector is way too big.
Consider exhaust velocity. The exhaust port is larger than the tube. So the exhaust has to speed up to enter the tube. Then it has to speed up even more to get past the triangle/venturi, and then it immediately slows WAY down when it hits the collector. Now if you have less than 2.5" exhaust piping, it must speed back up to get into the rest of the exhaust system. All this shuffling of velocities is murder on flow. Just say no. This is not a good setup.
A simple manifold at least gets gradually bigger as it approaches the exhaust system so that when you reach the pipes you've been gradually slowing exhaust velocity, not increasing and then dropping it as the Mustang header does.