Yes you do actually get more gas sometimes when pumping slower which is especially true during the hot summer months.
When you pump gas faster it causes the release of gasoline vapors, which your gas nozzle pulls back in with a vapor recovery system.
So by pumping slower you don't release as much gas vapor so more gasoline actually goes into your tank instead of going back into the nozzle as vapor.
I always pump gasoline slower in the summertime and I have noticed that I do actually fill my tank more with less money than I would if I had pumped the gasoline faster.
Just don't pump too slow otherwise other drivers may get irritated at waiting for you to pump the gas but pump it slow enough to where the vapors won't be released as much.
Still pumping it fast if you're in a hurry is okay too.