2008 GSA 60,000+ miles.
Decatted it first, there was a noticeable mid-range increase, seemed to spin up easier. Noticeable but NOT amazing.
Fitted a Akro' rear can, sounded crisper but no difference in performance.
Fitted a set of Akro' headers with the can and it made a big difference, not just noticeable but a marked difference.
In my view the headers made the most increase... If the decat was one step up then the headers was like a further 3 step increase.
Hope this helps..
Roger.