[Hicompression]

The front O2 sensors (before the cats) control the A/F ratio. They do the same thing regardless of whether cats are in place or not.

The rear O2 sensors are the ones that check to see if the cats are working. If they sense that something is wrong they will pop a CEL (for emissions purposes). But, again they don't control A/F.

No tune is necessary for running catless (stock tune is fine). You pick up hp gains just from losing the restrictions of the stock manifolds and 4 factory cats. Will a tune pick up even more hp? You bet it will...but is it necessary? No.