Yeah what Bobby said. As for the O2 sensor, it's a crap shoot. I think some people damage their O2 sensors when they put in the DP and that may create the errors.

Recently I have heard someone say the high flow cats are TOO free flowing and they may throw the code at WOT.

I don't know the answer exactly, but I had my DP on for a good 9 months or so and never threw a code. I think some cars are just more sensative to it than others.