I think my friend does do windshield banners. I haven't asked him if he does, if you e-mail him I'm sure he'll answer your question.

I don't know of any "formula" off the top of my head. There are some formulas in some books out there ("the scientific design of exhaust and intake manifolds" to name one) but the formulas are always a little different.

And, the ratio doesn't matter. If you have a 75mm throttle body and a 4 inch exhaust, the ratio of the two would be good, but you are still going to be too big for our engines (a bigger engine, or one running boost, would be a much different story).

Also, 9D9, if your low end has decreased, you have lost that power (torque) at low rpms because the exhaust gasses aren't flowing as fast as they used to. Removing the restriction could do that at the bottom end, but it's still due to air velocity.

Having the resonator in place probably allowed the lower volume/mass of exhaust gas at lower rpms to flow well. Removing it improved the flow at higher rpms, but made it slow down at the lower rpms.