Stock thermostat begins to open at 18º0F/82ºc, and is fully open by about 90ºc. My car came from Japan with a 40mm Koyo aluminium radiator, and G-Reddy hard coolant pipes, so I have a feeling they fitted at 160ºF/72ºc thermostat, which would be fully open at around 80c - which is the temps im seeing.
I could probably only check this by removing the thermostat, but given the temps i see and the modifications to my car, im 90% certain that's the case.
So my next steps are to rectify the thermostat issue (new one), get a leak test done (to ensure no leaks at high pressures) and then to get a high flow cat to ensure its not back pressure causing more boost drop at high revs....