Well I switched to generic RAID-6 and added three additional 3-TB drives successfully, so I haven't had a data loss. Also switched to two link aggregated gigabit channels and jumbo packets (this is complicated to setup!)
Right now Blackmagic Disk test reports that I'm getting 100MBps read and write over the network (?!!!). That's pretty amazing and in fact is faster than my iMac (2009)'s local disk (80MBps) or my Mac Mini (2011) 60MBps. Although the MacBook Air (2012) flash drive is still faster at 200MBps. Still pretty remarkable since theoretically, gigabit ethernet can only give 125MBps. Of course, with our Windows 7 box running RAID-0 against a pair of Samsung 830, we are getting 1GBps!
The biggest limitation right now seems to be the transcoding for Plex applications. It doesn't handle 1080p well. Some say that going to 3GB of memory is the answer. It is incredibly cheap to do for $18: Kingston ValueRAM 1066MHz Non-ECC Notebook 2GB 1066MHz DDR3 I don't know why for a$1000 box, they just didn't do it for us. It uses an Atom dual core at 2.13GHz.
The other solution is to use the more powerful Mac Mini and have the NAS stream over the network as it is actually faster than the native drive. This is how I'm running it now, but nice to have a backup. If you replace the 1GB module that is already there, you can get up to 4GB in the system. But not sure if that makes any performance difference.
The performance link on Synology reports that they used to have issues with two memory modules, fixed in a DSM release and many of the speed problems have to do with loading too many packages into the system. So I'm buying more memory and turning off things.
Another thing to note is that if you plug an ethernet cable into port 1 and then another into port 2, then port 1 disables. Only use port 1 most of the time, unless your switch supports link aggregation 🙂 And to be clear, link aggregation doesn't mean you get 2Gbps, it just means that now you can have two client each seeing a full 1Gbps, so mainly useful for concurrent applications.
Final issue is your clients, the Intel ethernet seems good, Realtek seem not so good.