The posted 15 minutes of max MIL is for maintenace concerns only, fighters in WW2 never flew the entire mission at "full throttle" ,for one the gas consumption would be enormous and secondly the engines would require a lot more down time.
We dont pay any cost in AH for running at 100 throt the entire time, ir real life, you would be called on the carpet for it, in any air force.
So the 5 min is WEP, and the 15 min is a suggested max for full throttle MIL, thats all.