Isn't the white smoke at start up a probable sign of coolant leaking at the head gasket?
If your going to try to get 300000 miles life out of this, I would drop trans pan and change fluid and filter. I think the pump out change of trans fluid on cooler lines is better overall, but if fluid was never changed, that process has more probability of stirring up particles that could clog orifices
At a minimum take an oil sample and get it lab analyzed to see the current state and amount of steel and contaminates in it. I don't know of a specific lab, but the airplane mechanics send in the oil to be analyzed often so look in that service area.
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.