Your coolant temperature shouldn't rise no matter how many times you go WOT. I've ran my car on multiple back to back WOT runs and up to 235kmph without ever facing any overheating issues.
It is impossible to say what is causing this issue without looking at your data log but since you don't own a ktuner V 1.2/2 you will need to find someone who has one and record a log. This will give you a better understanding of what your car is doing when it overheats.
Based on experience I can tell you that there isn't any issue with the car's cooling system. The problem is with the tune. Your cylinder temperature is rising under heavy load; possibly caused by either poor quality fuel or running too lean. But again this can only be verified by looking at datalog.