RE your questions...
"Do you have the extra brace? I didn't see it on their site, but I read about it being necessary for the heavier machines. Do you find there's any additional noise at all with the shims? (Optional Change System)"
I don't have the extra brace. I purchased the extra set of shims. I use varying number of shims for the different machines and don't notice any noise.
"What's the table top made of? Is it a resin, or is it a painted wood or wood product?
How do you think the table would do with the 28 lb 15-91 on it, and a cat or two? That's my biggest concern... "
I think it's made of pressed board with a thick plastic coating. It's very smooth. It does okay with my 15-91 but I don't know how big your cats are??????????????? My DH says the table is pretty strong.