Here are some examples how the temperature influence the drain current in Ampere. (In this example for the Dual Cool NEX Mosfet from Texas Instruments)

At 50 C° ---> 100 A
At 75 C° ---> 100 A
At 100 C° ---> 95 A
At 125 C° ---> 80 A
At 137.5 C° ---> 40 A
At 150 C° ----> 0 A !!!

This one has a max operation temp of 150 C° the IRF ones has 175 C° , but they react similar.
And this is the reason why the temps of the mosfets affects the HW´s.
As you can see the critical case temperature is at something near 90 C° , then the drain current begins to decrease.
I hope this helps you a little bit to plan your modifications.