Alright, updated both Lumia 830 and HTC One M8 to the latest build, hard reset on all of them, here are my findings:
HTC One
-All apps downloaded like a charm, and promptly.
-No other problems at all, really.
If it weren't for the fact that my phone has a faulty processor (which starts stuttering ONLY with scrolling, for every single build and even on 8.1. ATT sent me 4 of these in a row), it would be my daily driver. Everything is perfect, and I'm especially loving the fact that the standard camera allows for recording in 1080p 60fps. There's options for 90 and 120 fps but trying to record results in the camera crashing.
Lumia 830
-Need to 'pause/resume all' multiple times for apps to not get stuck.
-Left it alone for ten minutes and it didn't even get two apps done.
-Networks services and a host of other necessary core apps refuse to update citing an error code that doesn't exist. Prevalent since last build
-Glance and other function come online after several minutes. Turns out the trick was having to wait for them to install in the background (there's no app installation in the store for some odd reason)
-If all these are the same, I'm inclined to believe the battery drain will be equally as horrendous as last build, especially with Messaging and Groove
Overall, I don't think the SD400 and 1 gig of ram can handle this adequately just yet. Other experiences with the 830 may vary, of course.
Gonna try it on the Lumia 1520, my mainstay device which has usually been reliable with Windows 10, and see how that fares.