You need to add an exhaust fan at the rear my friend. You do NOT have enough airflow with just a single output at the top. Your heatsink is also on backwards. It needs to be rotated 180 degrees so that the fan is blowing towards the rear exhaust fan. That is the shortest route for heat produced by your CPU to exist the case. On top of that I would upgrade the heatsink to a push/pull setup as well to further reduce temps.
Fans are cheap and you really cannot have too many.
IE: Rear <----||||----<--- Front
Probably why you saw little difference in temperatures. Orientate the HS/Fan properly with a rear exhaust fan and you should see a big difference.