It would be a little more efficient outside the body, but IME its not all that substantial. They still have bubble plates and you are only losing an inch or two of contact area. I run a Reef Octopus 200 space saver, and it has been one of the top performing skimmers I have had. One beating it was a recirculating one and ones it has beat are ATB cones and vertex Omega. I would likely buy it again even if I didn't need the space.
Cleaning is a bit more a pain in the a, but not all that bad. Its more complicated if you want to service just the pump, but I have always performed a full cleaning on all the skimmers I have had whenever I feel the need to clean the pumps. Given that most skimmers nowadays can have the body taken apart, there is only a couple extra steps. It probably adds 2 minutes to my cleaning time.