The FX-9K parts have been binned simply based on the silicon leakage characteristics.
They have significantly higher leakage levels than the FX-8K series parts.

This is mandatory as an average part with lower leakage characteristics would require voltages beyond either the specification limits or the manufacturing process limits.

A low leakage part draws significantly less current than a high leakage part but usually requires higher voltage level to reach the same frequency.

There are some ultra low or low leakage specimens (sold as FX-8K series) which can easily reach the FX-9K series clocks even at the same voltage levels, but they are very rare. The specimens with ultra high leakage characteristics are much more common and thats the very reason the FX-9K series is based on ultra high leakage silicon.

The continuous thermal design current (TDC) for FX-8K series is 110A while the FX-9K series has 140A.
Thats pretty much the actual real world difference too.

A proper low leakage speciment can do the FX-9590 clocks with power consumption of 150-170W while the actual FX-9590 on average consumes 205-220W due the high leakage characteristics.

My personal preference is FX-8K series > FX-9370 > FX-9590.

Each and every FX-8K series part is able to reach the same clocks as the FX-9590.
It is just matter of cooling and the motherboard capabilities (= VRM).

The frequency scaling of Vishera rapidly decreases when the package temperature (external motherboard monitoring) reaches 50°C.
At 60°C the scaling has stopped completely and above that it becomes negative.

Thats the very reason why the FX-9K series is rated for 57°C TCaseMax.

In order to do that in full load (Blender, x264, etc) you will need a custom loop.
No AIO let alone a heatsink is able to do that. During normal workloads (e.g. multithreaded games) and very low ambients it is possible thou.

The FX-8370E is definitely the right choice for normal use (conventinal cooling).
For extreme benching (sub-zero cooling) the FX-8370 suits better.

The FX-8370E LACKS leakage (never thought I would say this) and thats why it will be limited by the usable voltage range.
On average the FX-8370 seems to go up to 1GHz higher on LN2 than the E-model.

A stock VID (for base clock, P0 PState) of 1.3875V indicates that the CPU specimen has VERY low SIDD (static leakage).
The leakage level itself doesn´t make the CPU either a bad or a good overclocker.

A high SIDD specimen with stock P0 VID programmed to 1.2250 - 1.2750V will definitely be able to reach frequency X at much lower
voltage than your low SIDD specimen. However it doesn´t make the high leaking part any better.

The high leaking parts require significantly less voltage than the low leaking parts, but at the same time they draw significantly more current and heat up much more rapidly than the low leaking ones. The higher currents cause greater conduction losses which itself already increases the temperatures across the system. Higher currents also put significantly more strain on the motherboard VRM.

As said before the low leaking parts are more suitable for conventional cooling (air and closed loop water).
On high-end or extreme cooling methods (high-end custom loop water, phase or LN2) the high leaking parts usually have a slight edge.

ps. All FX-8320E, FX-8370E and FX-8370 are ULTRA low SIDD chips thanks to the newer revision.
And they do go like bat out of hell 

FX-9K series are binned completely different, so it is definitely not the same thing to run FX-8K series and FX-9K at the same clocks and voltage power wise. FX-9K series parts use dies with ultra high leakage characteristics.

Ultra high leakage parts can draw 20-30% more current than low leakage parts and thats usually the case with FX-9K parts.

The reason why I dislike the FX-9K series is purely due the fact that they consume significantly more power even at the same clocks than FX-8350 or FX-8370 for example. Each and every FX-9K part has ultra high static leakage (SIDD) characteristics which makes the current draw (and temperatures across CPU, VRM) significantly higher. Prior releasing the FX-9K series these parts were scrapped due the excess leakage and resulting power consumption.

So practically these chips came from the garbage bin 

Per AMD specifications:

FX-8370

4000MHz (P0) - TDC = 94.6A
3400MHz (P1) - TDC = 74.9A

FX-8350

4000MHz (P0) - TDC = 95.3A
3400MHz (P1) - TDC = 76.7A

FX-9590

4000MHz (P2) - TDC = 114.4A
3400MHz (P3) - TDC = 90.4A

FX-9370

3800MHz (P2) - TDC = 114.6A
3400MHz (P3) - TDC = 92.3A

High leakage parts can generally operate at significantly lower voltages than low leakage parts.
Their break down voltage is also lower. A high leakage specimen might burn out at 1.5V VDD_CR while a low leakage specimen takes 1.65V before releasing the smoke. In this case the lower voltage doesn´t mean the high leakage specimen would require less power (on the contrary) or be a better specimen by any means.

As always the variation between specimens within the same leakage levels is present, meaning there are good and bad specimens in both low and high leakage categories.

If the dissipated power is low or otherwise manageable and there are no other limitations (such as the VRM or the CPU package itself) there is no real reason to avoid the specimens with a high leakage characteristics. This is obviously not the case with FX CPUs which are already running at the limits of the VRM, package and thermals in general.

Let´s use FX-8370E as an example.
Do note that all FX-8370Es are ultra low leakage parts in the overall FX scale.

Extreme Low SIDD specimen = 3300MHz (P0) VID Fuse == 1.28750V
Extreme High SIDD specimen = 3300MHz (P0) VID Fuse == 1.07500V

They both are rated for the same power so you do the math how much more current the "Extreme High SIDD" specimen consumes 

FX-9K series isn´t exactly a "cherry picked" bin but the contrary actually.
Chips which otherwise reach the 4.7GHz base clock but have too high leakage to be sold as FX-8K series will become FX-9K series.

IMO FX-9K series are for people who cannot overclock but have the money for high-end cooling and motherboard.

The newer FX-8370/E and sometimes FX-8320E are better than FX-9K CPUs.
They usually reach at least the same clocks but are able to do that with significantly lower power consumption due the massively lower leakage.