Firmware at test time was OS 6.1.1. I ran our standard NAS test process on RAID 0, 5 and 10 four-drive volumes. The Benchmark Summary below shows pretty consistent 100 MB/s-ish Windows File Copy write and read performance for RAID 0, 5 and 10 modes.
NETGEAR RN516 Benchmark Summary
NASPT File copy writes pretty much bump up against the limits of the single Gigabit Ethernet connection in our testbed, with all showing some write caching in their > 125 MB/s results. NASPT File copy reads run about 35 MB/s lower than writes for all three volume types tested.
iSCSI target write and read performance to target created on a RAID 5 volume ranks right up with other NASes with general purpose dual-core processors with 99 MB/s write and 94 MB/s read.
Your best option for attached backup looks like eSATA with an NTFS formatted drive, which completed the test with a chart-topping 128 MB/s rate.
Performance - Ranking
The RN516 is classed as uP-2 in the NAS Ranker (dual-core general-purpose CPU) and it's easy to find since it occupies the #1 spot.
With such a high ranking, it's tough to pick out the 516's weaknesses. Lowest ranking (#5) is for the NASPT Video Playback tests and RAID 5 File Copy reads for both Windows and NASPT benchmarks rank at #4. But these are mere quibbles. Looking at the results as a whole, the RN516 is a top performer that manages to hold its own even against some quad-core competitors like Thecus' N10850 Top Tower.
NAS Ranker Performance summary NETGEAR RN516
NETGEAR has produced a top-performing NAS with its ReadyNAS 516, but with one key weakness. Its performance cries out for a 10 GbE option, which it currently doesn't have and which NETGEAR has given no sign of having anytime soon. If you need 10 GbE, your alternative is one of Thecus' Top Towers, which come in six, eight and ten bay flavors. If you'd rather go with aggregated Gigabit ports, the Synology DS1513+ and DS1813+ have four of them, which beat the 516's two.
If expandability is your concern, NETGEAR has you covered with its EDA500 five-bay expansion chassis, of which the 516 can handle three, one more than the aforementioned Synologies. You'll get no price advantage for NETGEAR's expansion box, though. At a bit under $500 if you shop carefully, NETGEAR is going for parity pricing with Synology's DX513 five-bay expander.