My bike, sitting on the side stand currently has 138mm of clearance between the tire and under seat pan. As stock the standard Street Triple has 126mm of rear suspension travel, the R 130. As stock many who don't spin the clamps around have the Triumph ST3 rear hugger hit the band clamp bolts at the mid pipe split, so not much room to work with under there.
What I'm saying is that there is barely clearance for the pipe let alone a muffler under the tail. Yes I rotated the clamp to get clearance but it really didn't stick out much. If you want a single muffler in the center then you'll want to swap the rear subframe with one off a Daytona.