I was in a small FC for a while and couldn't clear turn 5 because we lacked the number of skilled players to do it. Eventually I joined up with a very large FC and keep up with my old FC friends through LS (we already had a completely redundant linkshell anyway). It took me way too long to jump ship. Once I did, however, I found that nobody in my old FC cared that I left (I was playing much more often than them) and we still do things together anyway. Now, however, I can immediately ask a group of 300 people if they want to go run something. Not a day passes without somebody asking about running turn 5 or Titan EX or nearly anything else. Tons of people who have completed it before gladly jump in and help others get a clear.

There's virtually no downside to joining a large FC. People pretty much automatically help you because you're part of their group. The biggest downside I've seen is the constant FC chatter can distract me during boss fights sometimes.