I am new to the 914-6 but I was told all this stuff was Porsches first attempt at emission controls. I believe the deceleration valve was intended to slow down the throttle butterflies from closing too quickly or all the way allowing for more even burn of the fuel in the carbs on deceleration reducing unburned fuel from entering the combustion chamber. I would imagine if not working properly it could cause a high inconsistent idle.
The oil tank breather is another way to get the crank case fumes burned by the engine from the positive pressure created by the cooling fan.
Charcoal canister is another one they wanted to get the fumes pulled out of the tank and filtered.
I don't think any of them are a power robbing issue compared to the mid 70's cars.
I kind of like the idea of the fumes getting sucked up.
My wife says these old cars are all stinky!