Gepetto, I'm wondering... what was the air temperature on this day? Also, the thermostat is probably set to open at 83c So, unless I've missed something, your car should run at that temperature as a minimum. Here in Oz (South Australia) we worry when the temp of the car approaches 100c. For those of us old enough to have run cars with water in the radiator boiling point was an issue! However, modern coolants are totally fine up to about 120c. On a hot day (43c Low humidity) I have run my GTV4 at 105-110c with no problems. Though you do need to keep moving. As a cyclist I can assure you that the temperature of the road surface in these conditions is around 60-70c. However, turning on the aircon will add heat: lots of it.
The shrouds are definitely a problem. As would be a failed fan. The air con condenser was installed at an angle in my GTV4. It was a very effectively air blocker.
Have you checked: timing? mixture (a little bit lean will really push up temperature). With regard the GTV6 there are lots of places that air can get in after the MAF and before the chambers as there are lots of rubber parts that routinely perish. This was my first challenge. I have replaced virtually every hose and rubber seal. I'm in the process now of doing all the rubber and gaskets on the inlet tubes. Do you have one of those infra red heat sensor guns? They're like a laser pointer but they measure temperature. I was recently (and still am) dealing with blockages in the idle jets of my GTV4 and i can tell which cylinder is lean or rich simply by the exhaust manifold temp. At 280c it is lean (ie the car is running rough because the jet is blocked again). At 100c it is rich. Check this. Lean = Hot.
A list of leak sites from the MAF to the cyclinder:
The Snorkel can have fine cracks in the bellows or around the junctions with other pipes.
The other pipes can have hidden cracks or just be perished.
The clamps can be loose.
Check every joint, connector or seal on the plenum (there is at least 7 places for air to get in)
Under the Plenum the rubber connectors to the intake runners perish.
The clamps need to be tight.
The intake runners have seals that might be leaking. In my car the nuts on these were not tight. They were done up but way too easy to undo.
The injectors have rubber seals.
You get the picture. I'm an amateur but i'm fairly sure that you can have an OK mixture at idle but when running at speed go lean due to an air leak that opens when the intake pressure is low (ie sucking lots of air).
One final comment. You guys in America make lots of comments about it being really hot and humid. Humid helps heat transfer but not evaporation. Hot humid days conduct more heat away than hot dry days. Cars don't have evaporative cooling like us humans... just saying...