It will be pretty unbalanced in wars if 1 gang has 20 members while the other gang has 5.
Just create a second gang, and call it the same thing, you can gang hop between them if you like. Maybe some sort of alliance feature will solve stuff like communication between you, instead.
//Formidable - DeathWish MGS
We still feel 10 is an optimum size... increasing the number of gang members will result in heavy imbalance in power.