The FX-8150 is not really comparable to the processors of today, in a bad way. Although it is "8 core" it has core pairs which share a floating point unit so for any application that involves floating point units, such as most games, it effectively only has 4 cores. This means that if you disable 4 cores that make up 2/4 pairs you are likely forcing X4 to run on a dual core processor which will impact performance significantly. If you disable 1 core from each pair then performance should not really change at all, but neither should power consumption unless those cores could not idle efficiently. There was a reason these processors nearly bankrupted AMD...
For more modern AMD processors I would generally recommend adjusting the power limit if you are concerned about thermals. This would give applications access to full boost speed and all cores as they want while letting the processor governor manage balancing the power draw. Idle cores use very little power and in power limited situations, such as all core workloads, the cores will boost to a lower, more energy efficient, frequency. The only time disabling the cores could give better performance is if the application has very bad scaling with core count to the point that the gains from all the extra parallelism it is using are less than the gains from slightly higher core clocks.
With modern AMD and Intel CPUs you should not even need to worry about thermal throttling since they are designed to deal with it for extended periods. As long as the cooling solution is reasonable (good contact, removes some fraction of the heat, fans not obstructed by a solid sheet of glass) they should just work and automatically drop their power usage to match what the cooling solution can remove. How good this is for processor longevity is another matter, one I do not know a definite answer for, which is why some people might prefer to lower the power limit.
Arc is later than late as predicted...