According to Plex they now support hardware transcoding on Synology 64bit systems from DSM6 onwards. it's only for plex pass users, but possibly that might sort things out?
Want to try this with my system, but first need to go from DSM 5.2 to DSM 6, still havent pulled the trigger.
Not sure though of it supports the 916, the 3615, or the 3617. I thought it was the 15, but can't find the info anymore.