Yeps, I know I can change it manually via the console - this works fine in SP.
I haven't tried MP yet, so cannot confirm this, but I think someone from the devs commented that cheat mode needs to be on for the console command to work online (if it does at all). This is why it is a topic of concern to me.
I took a quick video of FPS numbers with 65 vs. 90 FOV. I don't see any issues performance wise - my FPS actually goes up ever so slightly when increasing the FOV. That said, I can only speak for myself, and someone else might have performance problems. Sorry if the counter is a bit blurry, but you should be able to see it on a PC monitor.