As an exhaust engineer, I don't agree with any back flow from Cat upstream, there is always a positive flow at the cat. The leak COULD do 2 things, burn or melt things in the path of the escaping gas in the engine compartment, or suck in extra oxygen from the pressure pulses which COULD cause the Cat to run higher temps from the increased oxidation. However after 200000 miles that is 100000 miles past the required life of the noble metals, so I don't think there is enough left to even do normal conversion. If your state has emission testing, pay your $35 and have them sniff the tailpipe just to get a quick reference. But if your going to run for another 100000 then get it fixed for safety and noise aggravation.