Wondering how the OP ended up making out with this problem?

 

Some thing is fishy here I did the duals and the only thing I hear that you did different is the diverter plate at the y my muffler man put a disc in there sending bout 60% to the right 40% straight mine is very even at the tips, in addition we replaced the resonator with a 24" glass pac to quiet the sound so If back pressure was the problem I should have had some and I dont after 5000 miles still good to go. Thinking the cat is suspect any weird smells??