one thing to be aware of when looking for a sound similar to the X6, is that they use a twin-turbo engine which will always sound better then NA.
back to Mazda...
if you are wanting HP, then you are going to have to redo the exhaust from the catalytic converter all the way back.
if you are looking for sound then it can be done with mufflers, but nothing is guaranteed nor direct fit bolt on.
the most HP gains will be appreciated with the use of a dyno and testing a variety of pipes and styles. now, i know that it would be cheaper and simpler if we were able to call up HKS, Borla, or Flowmaster and get a direct fit, but as with everything else for our car there is a lack of after-market support.
this isn't to say it can't be done, but we will have to infer a few things and do a little leg work.
Now, some of the research is simplified if we look outside Mazda. Remember that our engine is very similar to the V-6 used in the base Mustangs. so checking out some of their forums and after-market parts suppliers, we can come up with a reasonable exhaust design for performance and sound.
Naturally, custom fabrication and fitment would be required, but in these days of "Tuner" shops and "Performance Haus" places existing that do complicated cross-platform engine swaps, finding someone to fab up an exhaust would be easy.
