I'm not sure on the NetInstall Image Creator side of things. If you are looking for alternatives, I have found that using AutoDMG to make a base image and then AutoCasperNBI to make the NetBoot Image works very well and is working in our environment with Sierra 10.12.2.
We're using AutoDMG and AutoCasperNBI to deploy 10.12.3 fine but in my experience using Casper NetInstall Image Creator resulted in a 6 GB NetBoot image and now with these other tools it's over 9 GB. That makes a big difference when you are trying to image a lab of 40 Macs. Hopefully there will be a new version of Casper NetInstall Image Creator soon. And I always prefer using one tool rather than two. 🙂
@martinoc The actual size of the NBI from NetInstall or NetBoot doesn't matter.
But you should be able to check the "Reduce Size" box to get it under 8GB in AutoCasperNBI.
The speed difference is the way that NetBoot often creates a shadow file on the NBI server.
This can be sped up in by checking the "modified rc.netboot" option in AutoCasperNBI, which should stop the NBI's creating the shadow file on the NBI server.
Really, other than the underlying tech differntiating.. the NBI size doesn't matter as the client doesn't load the whole NBI.. only the bits it needs.
Apple published a support doc on how to download Sierra. https://support.apple.com/en-us/HT208202
You can still download it through the Mac App Store. https://itunes.apple.com/us/app/macos-sierra/id1127487414?ls=1&mt=12