Using mods in multiplayer games is a contentious issue, especially if those mods are completely cosmetic. With the exception of a few games, field-of-view adjustments aren't going to bring the house down. Unless it's GTA V, in which case Rockstar might just ban you for two weeks, if you happen take your enhanced peripheral vision online.
Sure, if you're flying around, spawning money on people, you might want to take your antics elsewhere, despite how altruistic your intentions are. But tweaking a graphics setting? That's on the more debatable side.
Over on Reddit, a thread entitled "GTA Online Ban MEGATHREAD" contains stories of player bans, ranging from the justified to the slightly more mysterious. Obviously, honesty is an issue when it comes to these things, but there are more than a few reports of online patrons being handed suspensions for the use of a FOV mod.
Here's the email Rockstar is sending to suspect players (via aust_b):
Receiving generous donations from players who are cheating is enough to warrant a ban (which is fair enough), so Rockstar is able to discern when someone isn't breaking the rules themselves, but has benefited from another who has.
I guess the message here is to turn off any mods you might be running before switching to multiplayer. Oh, and don't accept cash from levitating strangers.
GTA Online Ban MEGATHREAD [Reddit]