CPU upgrade to i9-9900k sucessfull but cpu is overheating

I just did upgrade of 2019 Imac 27'' 5k from I5 to i9 9900k and all is working. But CPU temps are just to high and it seems it is throttling too much, I'm afraid I will need to downgrade to i7 or back to i5. In this moment with i9 under load temps are constantly between 80-100C. Cpu temps on idle are between 50-65C. Does anyone have same experience? Maybe I messed something with thermal paste. If someone did this upgrade, what are your temps on CPU?