On my 2013 TriGlide I ran the E Series exhaust and had no cat in the header and it definitely is not loud. A bit louder than the stock pipes on my 2018, but still very mellow especially when going down the highway. I had Street Cannons first and hated them (too loud for me).
I'd put them on the new trike but I don't want to void the warranty just yet since I've only had it for about 9 months. You're right in that you can change the number of discs to help with the sound, but if I remember right, that also changes how much torque you get. I think it was less discs for more torque, but I'm old now and can't remember for sure.