I was curious what high-end mode did really, as the info online was pretty subpar. Some said it only increased LODs (level of detail) distances, while others said it increase volumetric fog density and other environmental effects. Having a good computer, I wanted to test high-end mode to see if I could run in all the time, or if it was only something for screenshots.
The rig I use to test and play the game is:
i7-4790k @4.0GHz Overclocked at 4.6
GTX 970 x2 in SLI
16GB 1600MHz DDR3 RAM
OCZ 128GB SSD
And resolution was 2560x1440
In all comparisons, high-end mode is the FIRST screenshot, while ultra settings with high-end mode OFF is the second.
All settings are cranked to maximum, including AA and screen filter. There is no colour filter.
No High-End (Ultra)
First obvious difference is the LODs. On the closest building (not the wall), the high-end mode version shows some decorative shields and other livery from further away. Minor props in the upper and lower streets, like barrels, crates and some scaffolding, also appear in the high-end mode while not in the ultra settings. NPCs appear as black blobs from far away at the same distance in both.
More of a subtle difference in this comparison; the main things to notice are crisper shadows at a further distance in the high-end. Look at the cliff face on the left side - the shadows on the rocks are darker and more realistic in the high-end mode than in the regular mode.
The main thing again is the LODs. On high-end, we can see the large wheel on the left-side cliff far off has detail such as scaffolding and support struts, while the regular mode does not. The high-end tree shadows look more crisp and realistic. To the right of the image, the shack on the road, we can see barrels and other small props in the high-end mode that are not there in the regular mode. Also, while taking the screenshot (hard to see here), I noticed the fog was more dense and produced a generally darker environment in high-end mode.
Grass Density Test
No changes other than some minute lighting differences - but I attribute that to the game's natural day-night cycle.
Combat Performance Test
No screenshots for this one. In high-end while killing mobs, my standard ranger rotation (evasive shot, etc.) gave me a low of 42FPS and a high of 65FPS. In non-high-end mode, I receive an average of 55 to 120FPS in combat, depending on the number of particles on screen.
Keep in mind, I tested around Keplan, a relatively low-pop area. In Calpheon or Heidel, your FPS will drop even further (usually in half or more).
On average, I have 90-120FPS in regular ultra, with about 35-45 in towns. In high-end mode, that was cut to a measly 55-60 average in the world (non-combat), about 50-55 in combat and I'd estimate around 15-25 in towns.
Unless you have a 980ti, Titan X, or an SLI setup, don't bother with high-end mode except for screenshot taking. If you have a beastly PC, you can run high-end mode all the time at 1080p with no problems (60FPS). It's when you get into the UHD resolutions, like my own 2560x1440, or even 4k, where high-end mode really hits your FPS. At those higher resolutions, it's up to you if you want to waver just below a consistent 60...keeping in mind that will drop in combat and even further with more players around.
For group PvP, disable it completely. NPC and player view distance does not change in either mode, so there is no advantage having it turned on.
Personally, I'd rather have a constant 60+ FPS than a slight visual increase that is hardly noticable when you're grinding, PvPing or in dense cities.