does anyone know what the difference in power is between Stainless steel and mild steel.

 

stainless is more cash and wont rust, mild is a hot roll or easy to get and cheaper

 

does it produce any more HP or Torque than the stainless......

and Mandrell bend and the conventional bending - is there and hp or torque difference between them

 

power is comparable between SS and mild. power with mandrel bent tubing is head and shoulders above crush bent tubing.

 

can you buy apexi N1 exhaust kits all the way threw not just cat back.....including extractors

 

Stainless Steel is also a lighter header than Mild Steel since Stainless is a stronger material. Also, since the stainless is made thinner, the sound will be a bit more of a "pinging" noise than with the thicker Mild Steel.
IMO, I would get a Creamic Coated Header which is Mild Steel with Ceramic on top. They're thicker, to keep the noise down a bit more, and the Ceramic keeps more heat outa your engine bay and also helps the exhaust run thru faster and smoother.