That’s a great question hardride.
Exhausts are few and far between as far as new systems are concerned.
Here in the UK I like a loud system. I have found with these twins that even with a loud system they can be ridden quietly.
Stock exhausts on the LTD and CSR models have quite a nice sounding noise but is neighbour friendly.
From my experience the BSM exhaust system is a very good aftermarket system. They are of Dutch origin and I believe sadly are no longer produce. They do crop up on Ebay every now and again. The MAC system also sounds quite nice but they are let down badly with quality.