I don't have an R3T, but I know Jardine makes a complete system (header to mufflers) that really gives the Rocket a rumble. This is what I run. Triumph also has their accessory muffers (TORs), and D&D makes a pair of slip on mufflers for the R3T.
I haven't heard any of the exhaust systems on the R3T, but I have used the TORs for the Rocket Standard. They have a nice, raspy sound. I currently run the Jardine system, which I assume would be very similar for the R3T. They make a much deeper sound that you can even feel when you're standing by the bike. They do resonate some inside of a full face helmet, but not at all with an open face. I wear a full face most of the time though, and I'm fine with it and am very happy with the Jardine system.
The Jardines will require fuel map modifications. This can be done with a Power Commander, TuneECU software, or TuneBoy software.. all of which have "canned" fuel maps for the Jardine system. Triumph has their own map for the TORs if you wanted to go that route.. and I believe D&D's can get away with the Triumph TOR map as well.