Synology just released as new NAS, the DS3617xs. I want to let people know that if they were considering the DS3617xs over the DS3615xs that technically speaking there is only one difference in regards to Plex usage and that is the CPU. I own both units and have completely dismantled them for hardware comparison. Everything inside and out are identical with the exception of the CPU and cooler. Of course the motherboard is updated to handle the FCBGA1667 socket, but other than that they are the same. Yes the DS3617xs can run with 48Gb instead of 32Gb of the DS3615xs, but Plex does not need much memory to do any of its work. I think I max'd out at around 2Gb with a full load from Plex. I have 4Gb installed.
The DS3615xs uses a i3-4130 Haswell CPU and the DS3617xs uses a Xeon® Processor D-1527. Now several months ago I decided to upgrade my DS3615xs with a faster CPU. I found the best CPU to be a Xeon E3-1246L v3 Haswell. I could have shelled out another $400 and got the Xeon E3-1286L v3, but he spec were not that much higher and did not warrant the extra cash.
Spec compare here:
http://ark.intel.com/compare/91195,80916,75121,77480
You will need to upgrade the CPU cooler as well. I used the Noctua NH-D9L. Key here is the socket support (FCLGA1150). Now I could have gone with a i7-4765T, but it lack the higher CPU clock speed, 3Ghz vs 3.9Ghz in turbo mode. Base frequency is about 1.5Ghz in difference. Even the DS3617xs' CPU has a slower base frequency of the DS3615xs by 1.2Ghz
There is about $1,000 difference with both units current availability and you can pickup the Xeon E3-1246L v3 for about $370 on Amazon and the Noctua NH-D9L for $55. Unit runs around 40C at full load with this setup.
Hope someone finds this useful.