yes people use foi on regular d2 realms to hit max ga ias bps with a rare circ, that doesnt mean its good. the problem with this is the fact that when they switch to their jav side they lose a light fury bp. most people arent really smart enough to realize the importance of this. they just see an extra 20% ias and think "omg its so good". they'd probably be better off using a dungos instead of the rare circ tbh. as far as the math behind the damage goes ill leave that to chef.
the only time Id personally use a foi is on something like a ward / diamond/ shadow bow faith with an eth 40ias jav on switch.
this allows you to not only use a rare circ, it also allows you to give up a 40/15 in one of the slots in favor of something like a 30ed/9dex/9str or 4rep/30cold res or 10 all res jewel or even a 30 max damage jewel if you wish since the ed min max bug doesnt apply in particular situations. Its been a long time since I actually tested the usefulness of a 30 max damage jewel tbh so i cant remember which exact build prefers it, but Iirc it depends on the amount of dex you use on your bowa.
as far as your statement about the dex goes and whether the stats are worth a loss of 40% damage, unless you are a glass cannon (in which case just roll a diff class cuz youre bad) the dex just gives an additional 60 life before bo like you said. BUT the whole concept behind a vita bow is that a dead zon doesnt do any damage so investing in your survivability may lower your dps but increases your damage done over the course of a duel and wins duels for you. as far as survivability goes your effective health pool increases pretty drastically with minor investments into your survivability on a zon which is why its ok to lose out on a little damage in exchange for some resists and life etc etc. so the point im making is that the stats are an indirect damage increase not a direct one. and honestly youd probably have a rough time calculating that because with damage its just a regular formula that has been provided that you can plug numbers into. the effective heal pool vs damage formula is so situationally based that most people dont bother to do the math they just try it out and say "wow i actually win with this". with that being said its such a good feeling seeing a creation of mine become the go-to build when for so long people were all about glass cannons haha. ah the good old days.
foi gmb is still shit tho. same with foi mb