I was able to get the muffler installed today and retune the carb. It took a bit more fuel on the high end to get her to sound right at WOT so I'm thinking it could have gained a little power as a result. I did not get a chance to cut into anything today though.
You say you keep melting the chain brake. Where does your muffler exit? Mine (both of them) exit the front but the two exhaust "tubes" have an angle towards the chain brake/bar but it hasn't melted anything. When in a large cut the front of the saw isn't close enough to the log to hold that heat on the chain brake due to the bucking spikes keeping some bit of space between log and front of saw.