I'm actually gearing up to use The Sprawl as a basis for a Shadowrun hack, and I'm trying to figure out how Essence and Magic would be affected by cyberware and the Synth stat.
The Sprawl did actually begin life as an AW hack of Shadowrun, until I realised that the bits I most wanted to emulate about Shadowrun were the non-magical cyberpunk elements. I did have a few thoughts though.
One important issue to consider is that Essence in SR (and Humanity in Cyberpunk) are essentially (hur-hur) game balance mechanics. As such, they're not needed in an AW hack, which is why The Sprawl doesn't replicate them in that way. What replaces them in The Sprawl is fiction-driving questions about cyberware in character generation and MC moves/principles/agenda.
It's implicit in the above statement that I'm not trying to make a point about humanity as essentially biological. The idea that we become less human as we accumulate prostheses is not what I'm going for in The Sprawl. The Sprawl is about the interaction of humanity and technology, and how that intersection changes both, rather than that kind of biological essentialism.
If you want to make such a point, or if you want to balance magic vs technology as a zero sum game then you'd want some kind of counter attribute to Synth.
I wouldn't bother with both
Magic and Essence. I would probably just call it essence and then model the various fictional elements that raised or lowered Magic in SR as basic magic or playbook moves (depending on how you want to model magic in general). So there might be an Initiate advanced playbook or move tree or the like to model different fictional situations.
Essence and Synth might either always sum to 1, or perhaps the combined totals could never exceed 1. So if one is +2 the other is -1, if one is +1, the other is zero.
At the moment, there's no mechanical link between your Synth rating and the amount of cyberware you have. I would suggest something like this: Essence starts at +2 and is decreased by the addition of cyberware. One implant, Essence goes to +1 (as currently written, everyone has one piece of cyberware, so everyone would start with +1 at most); two implants, Essence is 0 (this would apply to anyone who takes the Chromed move); three implants, Essence -1 (At the moment, Killers with both cyberware moves). Beyond that, it would depend on how much cyberware you wanted to see in the game. Does it just bottom out, or is there a mechanical trigger for Cyberpsychosis? (I forget what the zero Essence state was called in SR).
I would then have Essence be the stat that powers the various magic moves, and maybe add in a basic move that drives at the heart of that zero sum game. Something like that advanced Monsterhearts move that lets you actually make a real
human connection (I don't have my copy of MH to hand, sorry).
The main question you need to ask yourself is "what do I want Essence to do
in the game". In an SR emulation, the answer could be as simple as "allow Magic as a counterpoint to technology by requiring a deliberate choice between the two", but the answer to that question will guide the way you consider this problem.