Always top up the mag if you're putting BBs back in. Even if your gun is ridiculously efficient, the only thing worse than running out of BBs in the mag is running out of gas while there are still BBs left!
GBBs of all kind, using propane/duster/MAPP, fire with gas, but store liquidized gas in the mag. So you're pretty much guaranteed to have the same fps per shot until you're really low on liquid and the pressure in the mag starts going down. And that's barring any other factors, primarily, cooldown for shooting too fast or ambient temp.
As long as you're not playing below 10-15 degrees or so, and you're getting all the BBs out of the mag and it still locks back, you don't have to worry about the efficiency.