There is no tailpipe test for 96+ cars. Since you have dumps this is not a issue for you.
In Illinois, these cars get the OBDII test which means the hook up a code reader to the diagnostic port and check for MIL code and perform a 'ready test'.
The ready test check the status of the various sensors in the emissions system.
If you have more than 'not ready' check or the service engine light is on, you fail. You may or may not fail because of the off-road pipe but, I know several people that have passed running an off-road/dump system.
There is no OFFICAL decibel test either. A ticket for 'too loud exhaust' is up to the police officers discretion meaning you may or may not get a ticket.