I have been playing around with Nexus before core openings, experimenting with both greedy eco and weird cheeses based loosely on Has.
One of my recent builds is 15 gate - 16 gas - 17 nexus - 18 core, where you stay on 1 gas in the early game to either rapidly ramp up eco, or do 2 base a Chargelot immortal all-in around 5:00.
I'm not 100% certain of the value of 17 nexus vs 20 nexus, though as I know 20 nexus has been the standard for a long time.
It appears I can get the nexus much earlier than normal this way, get core started by 1:30, and still end up with more workers than a 20 nexus at the 3:00 mark, but I'm not 100% sure of the math.
Does anyone else have experience with 17 vs 20 nexus? Would you be able to articulate the advantages of one, or state definitively which is better economy?