Truck Exhaust Theory

[KaiserM715]

Is most everyone who has long tube headers and high flow cats getting CELs?

Yes.

Is the belief that this is caused by the faster flowing exhaust or possibly do to a slight amount of air in-leakage that might be aspirated at a joint before the O2 sensors?

It is because the cats are moved further away from the exhaust ports and aren't operating at as high of a temperature, if I recall correctly.