RN104 vs NV+V2 read performance and disk storage variance

I'm using ST4000VN000 * 4 on mine, also 5900 rpm. 7200RPM drives run faster but use more power and generate more heat and probably more vibration that could cause them to fail at an increase rate. Choosing NAS certified disks (WD red or seagate NAS HDD for example) that are on the Netgear HCL is the best way to avoid problems. Both manufacturers seem to have many satisfied consumers on those models.
Anyway, my disks go up to 180Mb/s (according to seagate, so count 130Mb/s ;)) each, so if the NAS was fast enough I should have at least 3 times that value (admitting that the 4th disk is only for data safety, not used for transfer). The fact is the RN104 CPU is weak, not able to cope with that. The gigabit network would limit me afterwards anyway.

I don't think you can tune the block size without recreating the partition (not sure). I'm not an expert on that subject but I think bock size is often determined according to the space you desire to address. The block size is meant to store files in small containers where the OS can easily retrieve them. if you set the container to a minimum your OS will have trouble maintaining the address database (or addressing the whole disk), if you containers are too big then you will lose space because you will "reserve" space but only use a part of it (if you have a block size of 4k with a 129k file, you'll use 32 blocks entirely and 1/4 of another, you lose 3k, with a 64k block size, you use 2 blocks entirely and 1/64 another, you lose 63K. Repeat for many files).
More over I think Netgear works with BTRFS guys to optimise the performance too, there may be some performance tuning in the choices Netgear made for the filesystem, recreating it with the wrong options can be a problem to that regard. Not to mention something like this is very likely yo make you lose support (unless you factory default).
So I would leave that alone and trust Netgear on that. Once again, I'm not an expert on the question so my explanation may be wrong but that's how I understood the thing, I usually only tweak mount options for filesystems.

Speaking of tweaks, I think Netgear did some filesystem tweaking (which I believe is a block size change) with 6.1.5 and that those tweaks required a factory default to be applied, did you do one after that update ?
http://kb.netgear.com/app/answers/detai ... /related/1

If you didn't explicitly select encryption, then it is not encrypted. You have, I think, to delete the volume and recreate one with encryption enabled to get it. Of course encrypting has a negative impact on performance that is noticeable on the ARM models.

Disabling snapshots consists in disabling continuous protection on each share, I think that's what you've done.

For the space available you have to take some other things into consideration :
1- the OS partition and swap take some space (I think this should be around 10GB)
2- there are two ways to count the space, MB or MiB => http://en.wikipedia.org/wiki/Mebibyte
3- then there is the filesystem that loses some space too I think.

I actually have 10.89Tb available for data storage (not sure if this is what you meant by free).

HTTP/HTTPS are required for admin access, I don't think you can actually turn them off. They are there to get you some other options I think and the "toggle" is here to stay coherent with the interface but I don't think the OS will let you turn those off.

I'd also like to make sure that you know how important a backup is and that the raid is not in anyway a backup. So you shouldn't trust your data to one device. (I think you knew that but it doesn't hurt to make sure, and your phrasing concerning your data let imagine either that this was to get us a better picture, or that you didn't want to tweak yet because you haven't any backup and are afraid of losing those).