Way too complex to code and probably not worth the effort anymore, since you hardly ever find a CTF room with 10+ players nowadays (because IA R mods are what made CTF a lot more fun). 5-10% extra exp to every player wouldn't hurt.
In my opinion, players should only receive the extra exp if they have a minimum of 5 kills and 5 deaths (or something similar) in order to prevent abuse such as doing nothing or rejoining the room when low on HP, and so on. At least this way, only the contributors gain the extra exp.
@Devs: If you are willing to consider this, please look into setting the
default CTF timer to infinite. It's annoying when the games end in 30 minutes due to neglectful hosts.