All FX cpus the minimum clock are 1,4 GHz ? 6300 and 8XXX are 1,4 GHz minimum.
"I'm running an FX6300 at 4.6Ghz quite comfortably, well within safe temp and voltage limits"
Which are the settings ?
There are several "safety" down-clocks. I'm not really sure which is which as it's been a while since I was pushing my 6300 on an M5A88m motherboard. The 1.4Ghz speed, if I recall correctly, is the Cool-n-Quiet (CnQ) speed which the CPU's APM will also use to keep it within it's TDP rating. That will engage even if an overclocked CPU is extremely well cooled but simply exceeds it's TDP for a bit of time. I believe it's also a throttle speed the CPU uses when the VRM overheats and signals the CPU to slow down. The 800 clock speed is the first of the true safety limits, a thermal trip. The CPU probably also has something called an HTC, or High Temp Cutout, which completely shuts the system. Although, the VRM overtemp may trigger the thermal trip speed (800Mhz) on some motherboards or maybe even the HTC.
If you're seeing 1.4Ghz frequently when it's basically idle or light processing it's just to keep a cool CPU and quiet the fans (CnQ), if it's working really hard it's the CPU's APM keeping it in it's TDP range.
This is probably the primary reason more processors (especially the 8 cores) and low-spec motherboards didn't burn up more frequently. Back in the day I remember reading about people gaming with 5Ghz OC's (these things are extremely
overclockable) on their 4300 and 6300's yet complaining about stuttering in games. Their processor was probably bouncing off the APM limiter and constantly down-clocking to 1.4Ghz.
Disabling CnQ will only disable it at lower utilization, there needs to be a setting in BIOS to disable it for the higher utilization, it's usually called HPC (High Performance Computing) which is enabled to disable it. If you don't have it then your CPU will try to maintain an average TDP of 95W as AMD intended. It's pointless pushing clocks higher than that because it will just throttle itself ever more frequently. If it does have it and you enable it then you have to be very careful as this is the way to burn up your CPU or VRM or both.
As far as other settings: I keep the BIOS APM setting enabled (that has nothing to do with the 1.4Ghz throttling, at least on my boards) as well as Cool-n-Quiet and Advanced C-states. You can disable them but all it serves to do is keep the processor unnecessarily warm when it could go into power saving modes at low utilizations. Any other settings are pointless to share since motherboards work so differently and CPU silicon quality varies so greatly. My current board (GA990FX-UD3) is a bit up-market with good heatsinking on a relatively strong VRM (certainly, there are much better). It has the HPC setting.
Cooling your FET's with heatsinks is a good idea. Also locating a fan to blow on them. I'd suggest checking their temperature with an IR thermometer. While FET's are safe at 115C to maybe as high as 125C it's always better to run cooler for voltage stability.
For you, I'd suggest setting clock speed to 4.2Ghz and then raise voltage until it stays stable for at least 10 minutes of Cinebench 20. Monitor clocks, if it's not throttling to 1.4Ghz you're still in it's average TDP. If FET temp is OK (no more than 90-95C for margin) push it a bit more to 4.3Ghz or even 4.4. Raise voltage to keep stable, of course. At 4.4Ghz you'll likely see occasional throttling to 1.4Ghz in CB20 but gaming shouldn't be much affected.
When adjusting voltage use this limit as a hard and fast one: never exceed 1.55V with a reported CPU temperature of 70C. But definitely maintain as low a voltage as will remain stable for 10-20 minutes of Cinebench20.
And BTW: use HWInfo64 or Overdrive for checking processor temps. FX processor temps are not accurate but HWInfo at least works pretty well as a reference and uses the same temps that Overdrive uses but without the rest of the baggage Overdrive requires. HWInfo provides a lot more useful information than Overdrive does while remaining completely out of the way when it's not running.
Obviously you'd want the lowest CPU temps possible. But reported temps of 60-65C tops in CB20 (5C-10C thermal margin in Overdrive) should be achievable. Or better, depending on CPU cooling.