The old gang war system was changed because of gangs getting destroyed. Not a good idea there. However, (mentioned before) gangs having their own weps may be good to loan to the others while in gang.
However, having the donation logs, gang member notes and permission lists are good. As far as the gang stat upgrades go I think it would have to be whereas if members use their own energy to strengthen the gang the bonuses should disappear if they are kicked from the gang. Otherwise gangs can just take members in, use their energy for the gang, and then kick them. However, if a member leaves the gang on their own accord then 1/2 or whatever percentage (if this was implemented) of what they trained to the gang stays there.