Adding rear exhaust fan will reduce the hot air entering your PSU and keeps it from overheating.
Though, to solve your PSU overheating problem, replace your top mounted PSU PC case with bottom mounted PSU PC case. With this, your PSU has it's own airflow setup, independent from the rest of the case. With bottom mounted PSU PC cases (e.g Corsair 100R), there's no worries your PSU can overheat.