A good summary! I can't believe why there isn't a lot more debate about this - especially the most famous youtubers haven't addressed this issue at all, which is definitely awkward. Anyway, this "feature" is really game breaking and makes infantry combat feel very unpleasant with 90 degrees FOV.
You didn't mention that the iron sights zoom a little bit more than RDS/Kobra/etc. They are still affected by this "feature" (fov scaling) though, so I made my own calculations approximately the same way you did yours (I also assumed that using RDS and FOV 60 gives approximately the same amount of ADS zoom both in BF3 and BF4). The result: using iron sights with 70 FOV gives you approximately the same zoom level as you get using RDS/Kobra/etc with 60 FOV. So the iron sights still don't solve anything, as, iron sights are usually very clumsy to aim with and 70 FOV in general is totally unplayable (at least for me).
Another thing that people don't usually consider is the ADS mouse sensitivity. I ran some tests using a ruler and doing full 360 turns both in BF3 and BF4. The distance I have to drag my mouse in order to do a full 360 turn in hip fire mode is set to be approximately 11 cm in both games. So I managed to get these comparable results:
BF4 --- Hip ADS FOV 90 rds 11cm 18cm FOV 60 rds 11cm 18cm FOV 90 iron 11cm 21cm FOV 60 iron 11cm 21cm FOV 70 iron 11cm 21cm BF3 --- Hip ADS FOV 90 rds 11cm 21cm FOV 60 rds 11cm 21cm FOV 90 iron 11cm 21cm FOV 60 iron 11cm 21cm
So in conclusion, the only group that doesnt affect this new "fov scaling" feature of BF4 consists of people who, in BF3, ran with 70 FOV and used only iron sights, and continue to do so in BF4 (my guess about the size of this group would be somewhere between 10 to 20 people in total...) - everybody else are affected by this change. However, the players who use FOV of 80 or wider really have a broken game, where they don't have any usable sight options, and this definitely sucks.
Also, here are the links to official statements of DICE about this being a feature:
https://twitter.com/Demize99/status/387116850200719360
https://battlelog.battlefield.com/bf3/forum/threadview/2955064766645593353/19/
