I'll chime in on the AlFe over SS question.
SS is very prone to stress cracking. Looks really cool and everybody thinks it's indestructable, but it doesn't like vibration. I have seen some exhaust installs that didn't last a year before pipes were falling off, broken at a clamp or mount.
The aluminum over steel attempts to fix this problem by using the corrosion resistant properties of aluminum and bonding it to a vibration resistant steel pipe. The draw back to this is that anytime you do something to a pipe; cut it, bend it, weld it, etc., you compromise the coating.
FWIW