Over here in the UK we have an exhaust manufacturer named Predator, which you can look up on the web.
They make a full 6 into 2 stainless system for the 1300 which does not have any "chambers" or boxes, just 3 downpipes into a "3 fork" then onto silencer each side and the bike runs perfectly well as standard with it fitted as I have had 2 1300's with them in the past.
The only draw back is the silencers dont look great as not same shape as stock.
I have also had a few 1300s with original 6 down pipes straight onto short "turn out" straight thru silencers and the bike sounds great with no drop at all in performance as far as I could feel.
Hear mine here.............volume up full as its a treat !!
Pete F
UK