I was able to ADB sideload (through /e/ Recovery) the 1.5.1-S-dev build yesterday on my FP3+ (running /e/OS 1.5-Q-dev until then and having the bootloader unlocked). The actual procedure took hardly more than 5 minutes. Be prepared the UI is quite a bit different after the upgrade. 
I lost my Calendar entries (they are still in my Murena cloud though), but Phone, Messages and Contact data all survived the upgrade.
5 Likes