So you bought a DP with a high flow cat in it? Remember, it's not enough under the law to just leave one cat in. The car came with three, so it has to still have three - in their original positions - to remain legal.
OK, I looked at the attachment. It says what you said. I agree that it shouldn't effect things like suspension. But again, if you don't have three cats then the car is illegal for highway use and at that point they probably have a leg to stand on.