You mean something like this???
Yes, it does affect the volume of the enclosure. I only have the one driver, none inside. I originally had it mounted the traditional way, but the driver took up too much volume, and as a result, the enclosure was too small and I was blowing subs.(This sub needs exactly 1 cubic foot sealed enclosure as recommended by the manufacturer or else bad things happen) Since I mounted it this way, it plays louder, lower, powerhandling is up and hasn't blown yet! (knock on wood).