There will be 3 subjects,

1st one will be about FOV.
2nd one will be about DPI.
3rd one will be why you shouldn't trust most pros settings.

Those topics will be theoreticals and backed up with facts.

1)FOV:

The consensus is that, when playing fast-paced "arcade" fps (or AFPS is you prefer), you better have to alter your fov's value and particularly increase the default value that is 90 (4:3) on most games.
This is where it gets interesting; there's actually no point to increase your fov's value above 95, in 2015+.
You will instantly tilt and say to me "retard, fov is only personal preference, and a higher fov gives you larger angles of vision, thus making it an advantage over "low" fov."
The thing is, you have to take into consideration that fov 90 on 4:3 is NOT the same than fov 90 on 16:9. The same thing for fov 90 at 1080p(1920) compared to fov 90 at 600p(800).
Basically, the wider your aspect ratio, the higher your screen resolution, the more angle vision you will get from fov 85/90.
There's also the vertical fov, that's prolly the only type of fov that won't change much from screenres and aspect ratio.
But we are talking mainly about horizontal fov there, as most people do care mostly about horizontal fov.

Now you'll ask me, "why would I stop using my 105 fov @ 16:9, I love the very large horizontal angle vision that it gives, I can see people coming from the sides of my screen earlier than cunts using low fov, this is clearly an advantage".
Yes but... No!
Yes, because obviously you can see more people at the same time with such an insanely wide angle vision.
But no, you won't get any real advantage actually, because the higher your fov, the worst your mouse will feel, in a sense that it will just make it both slower and less snappy, as well as less "pixel perfect" type of aiming possibilities.
The remaining downsides of high fov is that everything appears smaller, while the "camera motion" will feel kind of unnatural, as the whole screen has a deformation especially at the edges of it. Try some ridiculous values like 130+ and you'll understand my point better, and prolly also get motion sickness. Keep the same sensitivity, then try 90 afterwards. The camera motion will feel tremendously more natural and "round", so to speak; and making a perfect circle with your mouse will be easier.

Another thing that you have to know, is that 90 fov @ 4:3 is 90 horizontally, while at 16:9 it's actually ~106(!).
Basically with the current gaming monitors, you get much larger horizontal vision without even adding deformations at the edge of the screen neither giving "motion sickness" compared to an old CRT.

Fov 103+ made sense on a CRT, but doesn't make any sense on a recent gaming monitor.

To summerize, fov 90 on a current gaming monitor is actually almost the equivalent of fov 100+ on an old CRT, and on top of that, the screen deformation that you get from a CRT will be almost non inexistant on your recent gaming monitor.

2)DPI:

It's fair to say that mice that use 16 000dpi as a marketing value is so wrong at so many levels. We all agree on the fact that 8 000+ dpi is literally useless.
Actually anything above 3500dpi still doesn't make sense at all. Will prolly make sense on 8K monitors, but considering that 4K isn't even the standard yet... we got plenty of time.

Now the real deal, DPI actually matter! As long as you stay within a "reasonable" range.
Why does it matter ?
First of all, all mice with 3310/3360/3366/3389 etc sensors actually perform without any prediction/smoothing/shitsThatMakeSensorFeelBad at any dpi values from 400 to <3500dpi. Above 3500dpi, things are getting fucked up because of predictions added to the way the sensor behaves etc.

Have you ever heard of pixel skipping ? I guess so, but you might be unaware by the fact that it happens even at 1600dpi. Basically, whenever your sens is above 1 (value in quake/source engines), you get pixel skipping, to some degrees.
To have a better understanding of pixel skipping, you might want to try 400dpi with sens 8 and then try 3200dpi with sens 1. Zoom in, put a tiny dot as crosshair, and move your mouse very slowy on a wall. At 400dpi, the mouse will cleary skip pixels, while at 3200dpi, it will be pixel perfect. It's especially true if you play at 1080p+. It's especially true if your screen res is FULLHD or 4K.
Pixel skipping is altered by the relation between your sens value, your dpi value and your screen resolution.

Another example, 1600dpi sens 1 at 4K res will make it skip some pixels, while at 1080p, no pixel skipping at all. The solution then if you're playing at 4K ? Try 3200dpi sens 1, skipping will be gone again.
800dpi sens 1 will not skip pixels at 720p/768p/600p while skipping some of them at 1080p, and skipping a fairly large amount of them at 4K+.

It's almost always better to increase your dpi value rather than your sens value if you want a higher sens.
Pixel skipping is a thing, for real, and it happens way sooner than you might think.
Some people still think that 800dpi sens 2+ doesn't have pixel skipping and that it only becomes noticeable on past sens 5+. The reality is that THERE IS pixel skipping at sens 2+, but barely noticeable in most situations, at least compared to sens 5+.

"Now what?", you ask me.
I'd say, set your sens value accordingly to your dpi value and screen res.
Basically, if you're a very low/ultra low sens player, and you're not on 4K screen res, then 400dpi might still be an option, especially if you're still using 4:3 low screen res (CRT-like).
If you're on 1080p and your sens is med/low, 800dpi could be a solution... At least if you don't mind at all having little amount of pixel skipping.
If you're on 720p and your sens is low, 800dpi is the solution.
If you're on a high sens, 3200dpi might be the best choice. But never go past 3500dpi, and 400dpi is an outdated value that obviously can still work decently, but actually makes no sense in 2020.

3)Most pros don't know shit.

The reason why prolly 80% of the pros are still using 800dpi is only because there's an old video of a logitech engineer in which he said "anything above 800dpi is not possible unless you add "artificial pixels" which would result in more calculation from the sensor and thus making it less snappy, let alone accurate (which was true back in the days, but not true anymore with 90% of the modern optical sensors).
The other reason is because most pros haven't tried anything else than 800dpi and they don't see the point to do so, they play with what they're used to, for years.
All of these reasons + placebo and psycological effect "I'll play worst on 1600+dpi, and I feel comfy enough with my current 800dpi, so why change?"