Agreed, diagnostic tests are the best way forward.
RE: being on topic: The OP also asked about uprated Radiators, amongst other things, to which you offered the view that the standard Radiator was fine with his spec. I noticed the 'Autobahn 88 Intercooler' listed in his spec list and offered the opposing view that a standard ~15yr+ old Radiator (in most cases) is somewhat marginal in that configuration, as evidenced by my own experiences.
IMO, if you're replacing the Radiator and are running increased boost over stock, you are as well going with an uprated item with a larger coolant capacity, increased fin count and metal rather than plastic end tanks which in the case of many Toyotas of the era tend to go green and brittle before failing with age. Running a FMIC tends to show weaknesses with aged Radiators. Yes, a stock new replacement may keep the temp gauge in the right place but it's the same argument RE: stock or uprated when doing anything with these cars and in this case there's not much of a cost differential.
I think the very act of replacing the rad - done properly - sees a cooling performance increase as it should include a Coolant flush / replace, new thermostat and new rad cap.
All on topic and useful information for other members now and for future searches.