Surely the cheapest and easiest fix to ditch all the cats is- remove the under gearbox catbox, leave the right hand TOR, and make up a short dogleg straight through pipe to connect the header to the TOR - and abracadabra - and you have a 3 into 1 cat less straight through exhaust! Saves weight . costs nothing, might work?
I'm going to get a bit of flexi pipe or make up a short joint piece and try it for a test - nil cost, probably too loud , but I'd like to hear what it sounds like. There's a big HD rally near me in a couple of weeks so it would be kinda cool to not only blast them in to the heather but deafen them as well!!