I suggest limiting the balance to a maximum of same color on same team, ie. 5 or 10 ppl. I would loooove to understand the reason why clans are allowed to join the server with 15-20 people, getting on the same team thus using the casual non-clan ppl to farm XP for their members. It happens on a huge scale. It is organised XP-farm. I wouldnt be surprised if most clans on puplic servers even organise XP-farm days where the entire clan join the same server to boost their clan members XP wise.
In any other multiplayer FPS/TPS game its called ghosting and WTJ, and in cRPG its exploit.
I totally understand that you as devs want to support and encourage teamplay and socializing, but fact is that the banner balance is abused on a major scale. 15 people on the same TS/Vent can organise and plan their attacks. They have a huge advantage comparred to the rest. That is completely unfair to the rest of us who want to get a fair and equal match.
And to you clan-people: Far most of you have your own server. Go use it if you want to clan-play and stop exploiting the pulic servers. Yes, i call it an exploit.
http://en.wikipedia.org/wiki/Exploit_(online_gaming)