1997 Chevy Tahoe catalytic converter

I have a 97 chevy tahoe with dual exhaust back to the muffler so it has two catalytic converters. I recently installed cherry bombs for a better tone. the mechanic told me my converters were clogged and needed to be replaced. He said you must replace from the manifold back to the muffler or the service engine light would be back on within 6 months. This doesn't seem logical to me. Is this the truth or is he trying to make money off me? It is a $600 project!