JMHO: Without doing any internal motor work (cams and/or pistons) I believe that Carpenter's Brutes will make the most power, the available Aussie set ups (CES and Reband) will come in next place, and anything else that you might be able to source with a full header design will follow them. Jardines are no longer in production.
Just an exhaust upgrade of this sort will require cleaning up your intake array and a good, professional tuner unless you can source available TuneBoy or Tune ECU downloadable tunes for whatever you might choose to do. With just a quality exhaust, intake work and a good tune you can turn your R3T into a sneaky little monster.