Even at V5, it throttles down. I'm betting on the issue being the pi. Autotune drops it big time if I try to use any of the resources on the pi while mining. I'd set it up in my office and connect it to my antique AMD mining rig, but the temperature is still swinging up and down in New Jersey and my heat may kick on at some point and blow hot air on my equipment overnight.
Yup you are running into the problems I had with the Pi. A couple of pages back in this thread you'll see the test results I had with this. The ARM cpu that the Pi uses is a great little gadget however just isn't powerful enough to run these r606's at the right speeds. No matter what voltage I set it to, low to mid 400Mhz is where is always settled.
However if you have the newpac's those work great on the Pi. So what I did was ended up breaking down this setup into 2 different rigs. I put my newpac's and moonlanders on the Raspberry Pi. I have the moonlanders clocked at 720Mhz and the newpac's clocked at 400Mhz and they are working great. 8Mh combined for the Moons (have 2 of them) and for the 4 newpacs they are all getting about 90Gh/s each on a non-AB supported Pool.
I then moved my R606's over to a AMD FX-6300 powered Ubuntu Linux box that I have sitting around at home. It originally had a 8350 however the 6 core chip is a little more power friendly. And they have been running perfectly since. On Volt setting 5 I set them at 700Mhz and oddly it depends on the pool I am on to what final Mhz it settles on. When mining Bitcoin or Litecoin Cash, they will stay around ~700. One settles at 681 and the other either remains at 700 or drops to 698.
However when I switch to a Digibyte mining pool, they both settled at 644Mhz on this setting. Still got some reviews to see on this as I am unsure why which pool you are on would matter. I'm wondering if I had something kick off in the background that may have effected the speeds to downclock. *shrugs*
It's all still a learning experience for me and enjoying it. If you have any old PC hardware laying around, only suggestion is to build up a small Linux box and run the R606's there.
** Edit **
If you are still wanting to use SBC to run these, you can possibly try a Asus Tinker Board (same chipset however 1.8Ghz Quad core and 2Gb of DDR3 memory). Or possibly the Nvidia Jetson Nano. I've been curious about that SBC since it's designed mainly for AI development. However the stats on the board are quite nice. 4Gb of DDR4 memory. 16GB of onboard flash memory. Quad core A57 CPU (believe up to 2Ghz). Neat lil SBC for $100.