Are you expecting the heatsink temp to get to 80'C? That's hot! But even then (because of the fan forced airflow) the rest of the cabinet interior and parts should be at MUCH less than 80'C and not that much hotter than room temp.
It's always a good idea to do testing on high power heatsinks, that was the way I was taught to do it in industry. To just connect a pot etc to drive all your FETs and the PSU and turn it up to full power and measure how hot it gets for the given power dissipation with your chosen fan, then finalise the physical design based on that information. I would aim for max temp of the heatsink itself of about 60'C.
It's always a good idea to do testing on high power heatsinks, that was the way I was taught to do it in industry. To just connect a pot etc to drive all your FETs and the PSU and turn it up to full power and measure how hot it gets for the given power dissipation with your chosen fan, then finalise the physical design based on that information. I would aim for max temp of the heatsink itself of about 60'C.