EGR is not necessarily the problem many think it is, Removing the EGR on Blue and Black holden sixes cost you roughly a 100 kilometers per tank of fuel in that case the EGR actually cooled the cylinder stopping detonation because of poor head design.

Nothing is cut and dried when it comes to modified engines and modern cats offer very little flow resistance, more of a concern is to make sure the heat shielding is appropriate on the cat or one day you may park on the grass beside the road and see a grass fire start up under the car as cats often run around 900 degrees celsius temperatures.