Just for kicks and giggles (apparently it must have been a very boring morning/afternoon), I mounted the Intel stock cooler that came with my i5-4590 and ran Prime95's Small FFTs torture test. CPU at stock settings, mind you, so none of the 3.7 GHz all cores business this time around.
At first, it started gently at about 65C, but apparently the sheer amount of heat from the CPU eventually caught up with it and within 2 to 3 minutes or so, the CPU package temperature (and three of its cores) managed to hit 100C and ended up intermittently losing turbo boost (but never went below 3.3 GHz). Oddly enough, Core #3 (fourth core) never exceeded 91C.
Doing the exact same thing with a Cooler Master Hyper TX3 EVO netted me 62C "just started the test" and peaked at 72C after 20 minutes, maintaining 3.5 GHz throughout. So I guess you could say that even a "lowly" TX3 EVO is at least 28C better than the stock cooler under a stock i5-4590 in a "worst case" load.
Makes me wonder if the Intel stock cooler is designed to be just "barely enough" with no margin... How much do these cost Intel and/or their OEM partner to make? Especially since TX3 (EVO)s are dirt cheap everywhere I could find.