RN516 CPU upgrade

I agree, that's why I used "walking the line" to say it was hard to tell for sure and that I closed saying that Netgear should have found a way to get covered.

As for the CPU, my advice remains the same. As long as the warranty can be used via the first owner, I wouldn't open the NAS unless there is a clear need for more power, which given the CPU already present and the limited network capability of the device, shouldn't happen unless transcoding. I think the actual CPU can handle antivirus, encryption and file transfer at full network speed, which is the worst you can do if you do not transcode.
On the topic on the memory it was said "file Xfers, no real transcoding going on" viewtopic.php?f=65&t=70748#p432694
That doesn't justify voiding the warranty to me and even given the CPU price it's not worth it IMO. Either there is more power needed for some reason (oracle database, transcoding, photo/video editing...) in which case, this may be a good choice, or there is only the file transfer need and the difference won't be noticed at all.
If I were to upgrade something I would change the network card, that's the most likely limiting factor here but I don't think this is possible and it would require many clients to take full advantage of it (assuming that second hand purchase is for personnal use, ther probably won't be more than 10 clients). If it is just for toying around then I would spend money on a new TV, an xbox one, a camera, a professional firewall/network equipment if you want things you keep or for a trip, for a disneyland ticket if you have kids, VIP tickets for some kind of sport, a great restaurant with your wife...
If you don't have a backup system, I would buy a 316 and some disks with that money.

Just a rough price estimate here :
- CPU (xeon 1270) = 341€
- RAM = 51€ for 4Gb, you said 170 for 16Gb
- standard 10Ge card = 450€ (you need one at both end, and this supposes that the NAS uses standard PCI which I doubt)
- 10Gbit ethernet switch = 900€ (XS708E)
I spare you the cables and the UPS which should be around 150€ total.
total = 2311€ for 16Gb ram (without UPS). I may not have used the cheapest dealer around (materiel.net) to make the estimate but it is not the most expensive either and I took the cheapest things they had without thinking about compatibility so I may be under the real price.
Unless you buy all that, I don't think you'll see a real speed difference. For 900€ you could have had the same hardware in a PC with a freeNAS on top of it, not as good as readyNAS or readyDATA but good enough for most people.

As long as you are conscious that you are spending good money for something that you may not need, I have no problem with the upgrade. What I'm telling you is just that 2gbit/s is 240Mbytes/s no matter if you CPU is idle or not and that 4Gb is enough unless installing heavy third party software.

I'm not competent to speak about CPU replacement for those units, for PC usually updating the motherboard is enough to get the full support for all the CPU family, so there is not physical limit here, only software one and I know Debian (which will take what you throw at it) but I don't know the readyNAS bios nor the kernel modules they chose to compile in the kernel, so I can't say for sure what would be supported.