Ask Fox :) As long as i can remember he have used it.I use it myself to because i love fov 90 for shaft and rail but cant really handle a fov that low for anything else.I would suggest that you dont go overboard tho ...for example fov 140 on one weap and 70 on another.I think the contrast becomes to large then :P I use fov 100 on everything exept rail and shaft where i use 90.Sometimes i go upp to 105 on rocket but thats about it.

For me its an improvment but as usual its a personal thing.
I would suggest you use fox weaponbinds because then you dont have to worry about figuring out what sens you should have att diff fovs.When you play plus zoomed you can have the same sens at any fov and get the same feeling/sens.

i belive you already know this but just incase www.fojji.se :P

I have used it before, but I didn't like the changes when I switch between weapons.

it takes a while to get used to
i used to use different fov/sens/accel for each gun, but in some situations it doesn't always work out the best
say you use a really low sens/accel/fov for LG, and need to run away because you're being rushed/spammed
sometimes i'll just want to GTFO instead of thinking about switching to a different gun with higher sens/fov before i do so