If thats all your after you will be very pleased with the performance after you do the things you have listed. The s12's performance hike with those mods is very noticable. Its after that stage things get expensive and achieve less gains.
I really enjoyed driving one of my s12s with just those mods. It was great fun to drive and drift with lots of responce and a nice progressive power band. 12psi with improved engine breathing is very nice on the s12
There old cars so a good service with new plugs (bcpr7es spaced at 0.9mm) and new oil(10-40 or 15-50 or 10-60 synthetic) and filters(universal cone air filter and c109 oil filter) is your best place to start at achieving back lost performance.