Having said that, I've been guilty of similar worries with my Expert Electronics ColibriNano, which runs incredibly hot! I was so concerned that I wrote to EE seeking some reassurance and I was told very clearly that the unit is designed to cope with those sort of temperatures. Nevertheless, I decided for the sake of longevity, I would fit some heatsinks and after completing the task, I felt much better. The ColibriNano ran cooler to the touch.
So with that little success, I thought I'd do the same to my FX-4CR. Opening up the case, you can see that there's a large aluminium heatsink plate in the middle of the radio and the high-temperature components press directly against it to help dissipate the heat.
The hottest part of the radio is the very top (and slightly to the right side). This makes sense when you consider the location of the PA and so I decided to attach my heatsink there. It was a very simple matter to remove the top and rear covers, allowing me to apply a bead of thermal-paste to the internal heatsink. The parts circled in yellow are the areas requiring the most cooling.
With that job done, it was just a case of reassembling the radio and then applying the external finned heatsinks. I applied a couple of dabs of thermal-paste to the underside before spotting with high quality superglue, being careful not to get the glue on the rear edge (or I'd effectively be gluing on the rear cover, lol).
When the job was done, I ran the radio for 30 minutes or more, repeatedly sending a CW message into a dummy load at 20W. At the end of it, the radio felt pretty warm to the touch but there was nothing to worry about at all, so I'm 100% happy that there would be no issues with overheating at the power levels I normally work at.
I guess time will tell.
73, Tom, M7MCQ.