Of course. I did not study gas station efficiency with proper data. I do however, believe that I can make an educated assumption - given that I have been designing and implementing large-scale systems for over 20 years now - and efficiency is one of the things I deal with day in and day out.
Basically, unless you can ensure that there is at least one attendant per occupied pump at all times - you are creating a new bottleneck. It's simple math. The moment there is an occupied pump that is not attended by someone - you are in-efficient. Of course, if drivers can pump their own gas - you basically assure that every occupied pump has an available individual to pump that gas.
Another thing we know is that you are putting a burden on all the poor guys that need to use this station - for waiting and paying for a service that they do not really need. So - are we really creating a benefit here for the population at large? Maybe, maybe not. Hard to tell.