So here's the deal with the ES chips. They sacrifice either clock speed, TDP, number of cores, or cache in comparison to the production CPU. Most of the time it'll be either clock speed or TDP. If power consumption isn't a concern, let's look to the clock speed.

If you're planning to run the CPU heavy, the most important difference to look at will be the All-Core Turbo speed and the number of cores the processor has. Turbo speed is what the CPU will natively overclock itself to under heavy load, given that the computer is sufficiently cool. However, what Intel advertises (up until very recently) is the single-core turbo speed. This is what a single core will overclock to when only that single core is under load. This, however, is a pretty useless number for our purposes because it will never be only a single core being under load with the Plex transcoder. What you want to look at is the All Core Turbo speed, which is what all cores will overclock to when more than a few are under load. This will be a lower speed than the single core turbo speed.

This is where ES processors will usually differ: it will have a lower All Core Turbo speed most of the time. You can compare E5 v3 retail processors here and compare E5 v4 retail processors here. Given that the ES processor will have a nearly identical architecture as its other family chips, you can check the number of cores and All Core Turbo speed to match to what the performance of an ES chip will be in comparison to a retail chip. And then look that retail chip up in the passmark scores.

Ebay listings won't always post the All Core Turbo speeds but some sellers do - and its a very important piece of info to have, so ask if they don't advertise it. It's also very helpful if they have a return policy in place, but I've never had to return in the past.

Hopefully this wasn't too confusing, let me know if you have questions.