As of the moment, I've had no problems whatsoever with the water temp going crazy, even at the circuit doing lap after lap for about 20 minutes. The temperature is stable around 84 to 88 degrees Celsius. At the circuit it can go up to around 95. Running a few laps at lower rpms (above 2500) gets the temp down quite easily and painlessly. If it's still hot when returning to the pit, I just idle it for 5 to 10 minutes at the most. In stock condition, it used go as high as 110 after only 10 minutes of circuit running. So there's been no need to change the radiator after all! The intercooler is still stock.
One problem I did have with cooling was the following. Like most folks out there, I'd open the hood and at least check the quantity of water in the reserve tank and under the main radiator cap before checking the oil level. I prefer to fill under the radiator cap first and then add into the reserve tank to the full line. I kept doing this for some time that I was beginning to wonder why the water was not up to the cap itself while the reserve tank quantity went higher than the full line.
It was the shop's guess that water was going one way but not the other. Since the related hoses are under the air cleaner area, I had it checked when I had the cleaner box installed. There's a place where 3 hoses meet at a T-connection. Apparently air was leaking from there, which prevented water to go back to the engine when necessary, especially when hot. That's why after it cooled down, the water would go to the reserve tank without any problems.
Now in summer, the buzzer does go off once or twice. This happened only when starting the car simply because the water was hot and not because it was dangerous. All I needed to do was to NOT panic while I let the engine idle for a few minutes before the system circulated and cooled down the water overall. It takes about 3 minutes before the irritating sound stops. Like nothing had happened.