I just installed the same muffler onto my factory Nismo H-pipe set-up that I was running with my standard muffler. We had to modify the mounting holes to accept the OE (Nismo) H-pipe bolt pattern - the spacing of the holes in the muffler flange are approx. 1/4" off from the studs on the H-pipe flange.
The jury is still out - I was pretty accustomed to the sound of the standard muffler but I do like the deeper tone the S-tune muffler provides.
When I changed to the Nismo H-pipe set-up and still had the standard muffler, it gave me a small bump in mid-range power. I think I feel a very slight increase in (or an earlier onset of) torque now that I have the S-Tune muffler on the car.
Now people look at me funny because I keep bending under the car to admire the shiny muffler body.