Hey RG, brass = more mass = more sustain and for nut adds some brightness coz it's metal not bone or some kind of plastic. Ivory sounds great (more dense than bone) but not PC to use, then there's carbon nuts to reduce friction. I went down the brass path on my early Japanese Anson strat 35 years ago when it was the fashion... Brass nut, brass saddles, brass scatch plate (needs to be lacquered or turns blue.. good reflections though) does improve sustain, and I liked the sound of it (it's different, bright but kind of mellow at the same time if that makes sense). But do you want more sustain ? Sure some additional crispness I think you'd like (so brass nut ?). The sustain you notice more with big blusy/rock bends with lots of space, anyway my 2 cents worth.
The brass sustain blocks (where the springs plug in) in floyds (or similar) helps also. My Anderson's floyed has brass, and I recently bought a brass block for my Schaller floyed (in my tele), subtle but does improve sustain (also is warm ?? I think).
Cheers R