I use the Whiteline sway bar kit for LGT with solid links from kartboy. Another important item is the Cusco front suspension top hat with camber adjustment. These elements are the biggest improvement in my experience on the track.
Additionally, I just rode the Michelin PS5 1 month ago, they were even cheaper than the PS4 and it is one of the best tires I have tried on the track.
After that you have a lot of parts you can change, but they're going to give you minor incremental changes, nothing impressive like sway bars and more front camber. In example: Brembo 2 and 4 pot kit, cusco chassis bars, whiteline tie rod, LGT STI rear multilink bars, front & rear LSD diffs, ST MT 6 with DCCD controller, whatever you can pay...