Upgrade to better nas

Unfortunately you can only directly move the drives into another NV+ v2. This is because of the processor/operating system and a different raid format in the newer readynas units as well as a different filesystem in the latest range; which means the drives would not be directly transferable to any other NAS.
Keep the nv+ as a backup device and get a 6 bay NAS. I suspect you'll need 6 bays if your library is growing.
I have a 316 and an ultra6+, both of which stream bluray 1080p flawlessly to a Dune Smart D1. If you need to transcode hd material you'll need a 516.

You ought to have a backup of data on the NAS, re-ripping those movies would take a long time.

You shouldn't be having any problems streaming BluRay from a NV+ v2 to a Dune. BluRay requires a max of 7 MB/s, the NV+ v2 can easily stream at 10x that speed.

I think something else is going on.

One possibility is that an add-on is interfering with the performance.

Another might be that your switch/router doesn't support 802.3x flow control control. What model Dune do you have? Does it have a gigabit NIC or 100 mbit? What switch/router are you using?

I found with my ultra6+ that any additional sustained activity during streaming would cause stuttering. I had an NV+ doing a pull rsync from the Ultra occasionally, I had to stop it to allow decent streaming.
My 316 copes with this, was even able to stream during a volume rsync without a problem.

No doubt that the 316 is a faster machine than the NV+ v2.

I agree the doing rsync backup while streaming might create an issue with the arm processors. That is similar to the add-on theory - that some other process is loading down the unit.

Though even my duo V1 handled BluRay streaming w/o stress, even if someone was accessing some files from a PC at the same time.