The reason Megan Racing mentions for Off Road Use Only is mainly because there exhaust products increase exhaust noise which is a violation of California vehicle code section 27151 VC.
For the smog inspection, so long as these products are used after the catalytic converter, they should not pose a smog check problem. To be on the safe side, and to ensure your smog technician does not confuse your modified exhaust system for a pre-CAT modification, make sure to inform them prior to the smog test.
