I updated my iPad Mini 2 16 GB to iOS 10.3. I mostly kept it as a backup device and a eBook reader, so I figured it would be a good test device. I didn't time the process as I was doing other stuff, but I estimate that it took about 30 minutes to do the update. So far everything seems to be okay. I'll watch it for the next few days then if it seems okay and no one else is reporting issues on devices similar to mine, I'll update the iPhone 6+ and iPad Pro 9.7.
I checked the internal memory before and after updating:
iOS 10.2
- 12.24 GB Capacity
- 3.99 GB Available
iOS 10.3
- 12.57 GB Capacity
- 4.59 GB Available
Perhaps the new file system helped the iPad Mini 2 pick up and extra 0.6 GB of memory.