One thing for sure, a retune will not only keep it refined and easy to drive, it will be a LOT better than the hopeless stock tune in my opinion. This is the biggest benefit of a retune with everything else stock, not much power gains but HEAPS smoother.
A cat back will get you 4 -6kw and maybe 6 - 10 kwin total with a retune after fitting.
Air intake mods, have not seen anything yet that helps power, but there are mods that increase intake noise which may lead you to believe power has increased

I am planning a new product to cure this lack of decent intake side mods for NA
If those power and smoothness gains above are simply not enough to light your fire then read on
You can liberate easily >30kw with extactors and highflow cats combined with cat back and tune, substantial money involved but the gains are absolutely there, think "significant power increase" with super smooth driving characteristics.
This is truly a matter on money, pick what suits the budget and go from there.