Nice post, Hive account@kiebitz!
I am just in the middle of setting up something similar as I have 4 Pi3s lying around here.
Do you have an extra fan behind the rack or are the heatsinks visible on the picture sufficient to keep the Pis from overheating? The screenshot looks like you're running them on 100% CPU and - I assume - all cores?
I installed one in my garage today making use of the cold temperatures and it works: No heatsinks or fan and it never goes beyond 65°C running on full power. Let's wait for the summer... ;-)
At the moment, I BOINC with it just by command line (WiFi / SSH) but I plan to put the Gridcoin wallet on it to stake 24/7. Although one gets all necessary information, I still like the feeling of a GUI wallet view so I plan to set this up too and contact it via VNC.
And just to compare it to other numbers I've read about: How many GRC/day do they generate roughly?