How much power do you gain if you change the s pipe on a scion tc. i have a k&n intake, Megan header, and magnaflow exhaust. i want to know if it's worth it??

Youll get a few HP but nothing earth shattering. I noticed a smoother idle and a little more pull over 3000 RPM but nothing all that dramatic.

Will that s-pipe pass smog check in CALI?

All of them will since they do not modify the emissions system in any way...it would be like bolting on a axle-back