going from your sig, you've already got done exhaust, intake, etc = good start. Chasing more kw from the NA ends up costing more $$ for less gain, unfortunately.
window tint (if not already tinted) is simple/cheap.
kit before lowering - if the kit sits lower than stock, then you're asking for trouble doing it the other way around
at the moment I'm doing all lighting (replacing any sign of piss-yellow ). again, cheap and simple. for me its something to do whilst thinking of what ELSE to do.
clear/smoked indicators? gauges? strut braces? alarm/immobiliser?
on top of that, regular maintenance.