I saw a video, some time ago, on roasting a turkey upside down. The "pros," included the moister breast, as well as a more even overall cooking time. The legs/thighs need to reach a higher temperature for best results. Exposing them to the heat, from above, helps them reach "doneness," faster.
The recommendation was to allow the finished bird to rest for the requisite 20-30 minutes, then put it right-side-up, under the broiler for (I can't remember how many minutes), just watch it until it's browned to personal preference.