As always, you're a fountain of knowledge, Silva, but I just have one amendment ... I think in another post I think the jury was still out on whether the Stivo cat-back was of benefit to auto 1ZZ's. We only have one such case on record, though, so if another auto 1ZZ owner wants to give it a go, talk to Silva about the Stivo cat back he mentioned above.
Svalerko put a sportivo cat back on his 1zz and got gains proven on the dyno. I agree with you on the auto side of things buddha as an auto does drain a hell of a lot of power out of the motor. Maybe with an auto try the 2.25" diameter with a slightly more restrictive muffler to increase the back pressure a bit.
My thoughts on the matter but it could be worth a shot.
As for putting the stivo cat back on the sedan I've had a look and it does look to be the same floor plan as the hatch. Easy way to check is get the 2 side by side and measure the wheel base from stub to stub as the rest of the underside of the car is the same. But they do look pretty close. :D