Tuning wise, you'd have to spend a lot to get big headline figure gains but you can get better mid-range response, driveability and sound by mods like exhaust, hfcs, plenum spacer, etc but only if you then have the Uprev remap. If you search against any of those you will find lots of stuff.
Good luck with your search. The ST is a great car. So is the Z but in a different way.