Of course I got that joke :P
Okay. Disatvantage of all perks: Everytime you shoot there is a 99% chance that you die.
Srsly, what kind of disatvantages do you want? And yes, Weapon Master would be the upgrade of Weapon Expert.
Well nothing much. A perk that increases damage could mean less reserve ammo, increased bullet drop, decreased entry, some kind of tradeoff. I don't know.
And since weapon master is an upgraded weapon expert, other perks should get upgraded versions.
Like
Weapon Expert < Weapon Master
Ammo Dealer < Ammo Maniac
Ghost < Poltergeist
Handyman < Engineer
The same affects, but better values and harder to unlock, just like weapon expert to master.