I went through a phase of trying different bridge pins, but just as with not eating brussel sprouts and being led by your teenage hormones, it's something you grow out of when your balls drop.

In theory, brass should allow a different sound to tusq or other plastics because the saddle is a fulcrum point and you want everything that side to be solid with little or no give, or it takes energy from the other side.

I used to design vibrating structures (screens, feeders, tables etc) and out of curiosity, using available data for forces, masses etc involved, looked at it. All things equal, despite a brass pin weighing over four times that of tusq, the energy taken from the sound side with tusq is less than 0.003% different to brass and in real life is less than the bridge timber soaks up anyway. I doubt it makes a real life difference based purely on choice of material.

How snug it fits, how much the string ball tension relies on the pin? That's another matter and can explain why you sometimes see differences. If a replacement pin of the exact same dimensions but a different material were used, I would predict no difference. But as someone pointed out, to experiment you are changing many other variables by using new strings.

Just to the zero fret side discussion. I love them and when a guitar was made for me, specified it. The pros outweigh the cons. Go on the Fylde website and hear why they go on every guitar they make.