The reason you need backpressure is due to physics. The more backpressure you have in your exhaust, the more suction the exiting exhaust gasses create. This in turn PULLS exhaust out of your combustion chamber and sucks it into the exhaust manifold/header. The GT take-offs are the proper size diameter for duals. The 2.25" exhaust is large enough to flow freely, but is also small enough to create backpressure and improve torque. (Remember, horsepower is derived from torque and the more air in and out = more tq = more hp)
If an N/A v6 mustang
were to use dual 3" exhaust, then you would see a loss of backpressure. This would slow down exhaust gasses and not create the sucking effect out of the combustion chamber. You would see your upper hp figures increase slightly (above 5k rpms) but your tq would significantly decrease.
On FI cars, the large the exhaust, the better. I could run 5" exhaust out of my car and see more hp and tq than the 3.1" I have right now. FI cars need as free flowing exhausts as possible