I'm replacing the stock up-pipe very soon, that would be my only concern as far as the cat destruction goes. The other two I plan on replacing (or removing) down the road anyways.
I've heard such good things about Rotella T6, mostly from Subaru enthusiasts...I've got to go with experience on this one - too many people are praising it. so far I'm having good results from the T6 and hopefully Scotty's Cocktail proves just as successful.