So, with Denim on my AT&T 830 (as defined below) + 32GB SD card:
Software: Windows Phone 8.1 Update 2
OS version: 8.10.15148.160
Firmware revision number: 02179.00000.15213.43037
... from 8.1 update 2 (no restore), added win insider and updated to W10M 10549, then did a hard reset.
So far, so good - but the last build got slower over time so we'll have to see. Very snappy and very few issues - just a few updates hung and needed a reboot before continuing.