The cat is part of the downpipe. Unbolts from the rear manifold and at the other end where the resonator is. Not overly hard to do.
I have never heard nor had the notion to run lacquer thinner through any internal combustion engine. My gut says no as I'm not so sure the rest of the fuel system would enjoy it (orings, plastic parts,etc). Then again I learn something new everyday, soo...
If the cat is bad I'd just get a 3" ZZP catted downpipe and call it a day. Some PB Blaster on the 4 bolts that hold everything together a few days leading up to removal and away you go.
