Old pistons were made with brass, I believe, and stainless steel is much heavier.
Good question. I don't know, but it seems logical that they might.
No, they are heavier material, as mentioned above (although a larger bore horn probably needs a larger diameter piston at some point, which would also add a teeny bit of weight). I think the stainless steel lasts longer, of course, but it is also more dimensionally stable.