Their single core performance was weak; compared to the thuban it was almost equal. It was just the clock advantage that made the FX "faster". The opteron series, which came in 8 module / 16 thread CPU's, was weak due to it's low clock (2.3Ghz on avverage). This is why AMD lost so much marketspace into the enterprise market in the first place.
In order to get a FX platform running as it should; you defenitly need to invest in good memory, i.e 16GB with tight timings and high speeds (1600Mhz and above). If you wanted to overclock, you cant rely on having a budget board, but you needed a high end one. I had a crosshair Z and i learned pretty quick that the VRM was capable of over 250W easily, this provided all the headroom i needed. Now with AMD FX cpu's, there's a limit set by AMD of just 25A of current. You need to have a premium board to make the CPU use more then 25A if you want to pass or exceed certain speeds. You could test this very easily when you seem to hit a wall, no matter what voltage you throw at it. Now disable one or 2 cores, and when it does pass at the same settings, your running into a current limit.
Now overclocking with air up to 4.1 to 4.4GHz IS possible, but anything beyond that requires water and the higher you go the more you need. My CPU ran 781CB points (CB15) on 4.8GHz with a 300MHz HTT (FSB). 5Ghz was possible but it required a jump in voltage and thus much more heat then accepted. They where very nice CPU's and if you where lucky enough the CPU/NB would allow beyond 2700Mhz, which is amazing if you have such a chip in the first place.
People always focussed on multiplier overclocking. Setup a voltage and work from there. Really the best way is and has bin by FSB. You increase the overall speed of the system and because of that you would need less hard cpu clocks in order to archieve the same. Like, a 4.8Ghz / 300Mhz FX CPU is faster then a 5.2 or even 5.4Ghz using a stock FSB. But again you gotta be lucky that your system can handle it. Because of that advantage my FX platform really did well compared to other systems. It played every game and it worked out everything ive did with it (media, web etc).
However the moment ive replaced this with a 2700x pretty much bulldozed the whole FX all together, while requiring only half of the total system power and still be 2.5x faster. There's just a fundamental issue with the FX and it is it's shared module / threads. However now in 2020 you still see the FX holding really well; it's because the software ecosystem pretty much turned to more multithreading now, and even consoles are 'optimized' for 8 cores (jaguar). This will change tho as the Zen is being the primary ecosystem now for the new generation of consoles.
Really i loved it. Brew coffee, start at 01:00 AM and end at like 06:00AM with a well running system. AMD FX should be the basis for anyone wanting to learn how to overclock.