Ah, I see now. I wasn't sure if you were removing the cats or just relocating the O2's. That makes sense now.

You might want to try replacing the two main catalytic converters with straight pipes before tampering with the pre-cats and O2 sensors. The two pre-cats are already of the high-flow variety. The two main cats. are high flow as well, but don't have any sensors monitoring them that you'd have to worry about.