WTF is with people continually talking about "less back pressure" and "losing low down torque" on turbo motors with big exhausts??
This is simply untrue.
The exhaust wheel is more resistance and will be the limiter of "back pressure" on the head, the exhaust will not affect back pressure at the head.
On a turbo car the bigger the exhaust the better (within reason).
In answer to your question - different material exhausts sound different. as a general rule, if you keep the design of the exhaust the same, ie same diameter, same number of like resonnators etc,
1. titanium will be raspy and loud
2. stainless will be resonnant and mid volume
3. mild steel will be bassy and boom more
this is merely because the heavier material will damp the volume and affect the resonnace within the exhaust pipe itself.