It has nothing to do with back pressure. The diameter and length of the runners determine the resonant frequency of the exhaust system. As a result, the system will perform best at one and only one RPM. Increasing diameter usually moves the power up the rev curve. Unless you have access to expensive fluid dynamics modeling software, the only way to determine the precise outcome of an exhaust upgrade is by experimentation. And because experimentation is expensive, no shade tree engineer will ever admit they made a mistake. The only sure bet is factory stock, even though this is usually a compromise between sound and performance.

A straight six engine with typical 153624 firing order has equally spaced exhaust pulses in each bank (240 degrees), so there's no advantage to a crossover, h-pipe, or y. The characteristic straight six sound is like ripping cardboard when extended through parallel pipes. This is different from a v8. A 426 hemi has firing order 18436572. This results in uneven timing between pulses (270-180-90-180) on each bank, which explains that angry burble. So a V8 typically benefits from a balance pipe arrangement.

If you didn't lose any performance when the cat fell off, why not put it back?