Having ridden the R3T once back last December...I thought it was plenty fast the way it was. It's not as fast as the regular Rocket or Classic (or these days the Roadster) but it wasn't meant to be. I can't help you much on the pipes but the GIPro may be a good place to start. Removing the secondaries may help as well...though the GIPro may duplicate the same condition. K&N Filters, adding a turbo/supercharger would do it, I'm sure others will chip in.
But remember with the skinnier rear tire, the saddlebags, big windshield (though removable) it was designed to tour and hopefully give you a few more choices in the rear tire replacement category. And the torque. You may also get some added performance from tweaking with the ECU with a Tuneboy or PC...whatever number is the latest.
But if you really liked the "missed" power of your old Rocket...you probably should have just added Beetle Bags or Leatherlykes and tweaked that ECU for more performance plus you would have had more choices available for exhaust systems.
Dennis
36 dtg Dubai
37 dtg London
134 dtg Dubai
135 dtg Home (Triumph buying time)