The front 02 sensors are the computers input for a/f ratio. The rear 02 sensors are only there to tell the computer if the cats are working properly. The signal the computer sees from the front is a sin wave as the A/F ratio constantly switches from rich to lean. The rear 02 shows a relatively flat line, because the cats take most of the oxygen out of the exhaust (to make co2 and water from HC and CO). This way the computer knows the cats work. Without a cat (or with a bad cat) the rear sensor will mimick the signal of the front sensor, and the computer will know the cat is bad or missing.
The only effect that will be had from not having cats is that the CEL will be on for catalyst effieciency below threshold, IE: bad cat. there is no limp mode or anything caused by this condition, because of the fact that all it is is a catalyst monitor. So you will not hurt anything by running the offroad pipe without a tune. I have done it on numerous vehicles.
In a tune, they simply disable the rear 02 monitor, so that the computer basically doesn't care wether the cats are there or not, this is only to keep the MIL light off.