After upgrading to 24 GB the machine became very usable.
I wouldn't ever want to go back to 8 GB. Perhaps 16 GB might be usable for iOS and Xamarin development, especially if the Android emulator can run in some kind of HAXM [0] mode, but for ARM processors ... not sure if that's possible right now. I'm sure the SSD also makes a big difference compared to my Fusion drive right now.
Still it feels safer (more future-proof) if the machine has a bit more memory than 16 GB.
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[0]: https://github.com/intel/haxm/wiki/Installation-Instructions...