OP,
Below is the dyno chart before (bone stock Nismo) vs after (Bone stock with S-LINE midpipe)
As you can see the gains are definitely there, your rear muffler, albeit a Nismo, still is a restriciton. Yes it will taper down if you run the stock muffler. When we test drove the Nismo with the midpipe only, I honestly think its VERY similar to stock, if anything you gain a bit of bass in the exhaust note, it's extremely tame. As far as I'm aware, we've sold over 10+ sets of the midpipe since the release of it. From what I know, one person is running it with the Nismo muffler, but I'm not sure if she is on the forum or not
Here is a great picture, this is our midpipe mated to a Non-Nismo muffler
