Four characters following a leader and working in unison won't seem much different to another player than nine will. GMs will typically only get involved if you are reported by another player, so there shouldn't be any difference between 5-boxing and 10-boxing.

Mage teams seem to be the flavor of the month right now. It's a fun group, since they can pump out a lot of damage and sustain it pretty well and keep PVE mobs occupied with mirror images. I'm not sure how well it works for warlocks, my limited experience (leveled one to 85, am leveling four more which are at 61 now) says that warlock DPS can out-aggro the pets too easily for dealing with PVE bosses. For PVP, a group of warlocks sounds pretty awesome. Wasn't it Xzin who had a warlock team? Watching a mob or player's debuff window light up never gets old. Five DoTs... ten DoTs... fifteen Do-- whoops, dead!