Exhaust Popping

When does it pop? Mine pops a lot if I downshift in the process of engine braking. I believe it has to do with the fact that with less cats, sensors MAY be resulting in your car running a little rich (I have no cats). Anyway, from what I have read and been told, the popping sound is of fuel being dumped.

Unless it is doing it during full on acceleration, you should be fine. I just bought a wideband and an Anderson Ford Motorsport PMS, so I will be fixing any rich/lean issues - I am betting that the popping will be reduced a good bit.

AFM PMS:
http://www.andersonfordmotorsport.com/pms/pms-50.html
Allows you to tune pretty much every aspect of your 5.0's ECU. Air/fuel, timing, boost, etc.

if running rich, the excessive fuel can be burned in the exhaust.
When you use engine braking, manifold vac goes through the roof, so the puter dials back injector pulse (since the TB is closed) and it cycles the IAC and some other things (kind of involved for just one situation) for smooth deceleration under load. But it might just be as simple as that.

FWIW, the stock cats on foxes had 2 catalysts in one cat and a separate catalyst in the second cat (per side). Newer cats incorporate all the catalysts in one cat (same events still take place though).

good luck.