An to correct an earlier comment, the S-Pipe does NOT contain a catalytic converter. The secondary cat is located at the head of the Mid-Pipe. The S-pipe is usually separate from cat-back kits and only part of one header system that I know of. Yes, the S-Pipe is the most restrictive component after the header. Correctly built S-Pipes do not throw a MIL (Malfunction Indicator Lamp). Replacing the header with one that does not have the primary cat will.
Read about it here in our Tech Article section.
If you do run into a MIL,
then you can follow these easy instructions to make and install your own CEL (MIL) elminator.