The way one properly swings a heavy hammer is basically to lift it overhead, use the shoulders as a point of rotiation, and let gravity aid you when you swing it down. And I can easily believe that a lead hammer is much more devastating due to the sheer impulse even if the material is relatively soft and the impact thus gets slightly cushioned.
Then they would have made them out of lead in the medieval period. Lead was a whole lot cheaper than steel.
Most warhammers simply weren't like that at all, anyway. They were much more lithe, with smallish heads, and a pointed tip on one side.
And seriously, taking time to lift this giant, superheavy hammer directly over your head would probably be one of the worst tactical moves you can make in a battle. You'd leave yourself open to attack and could probably be knocked down very easily.
This is that this should require a lot of strength and exert you very quickly, and currently, while I can't seem to properly nail down wether faster exertion happens, there doesn't seem to be a problem with wielding heavy stuff with low strength, which is probably one of the main issues, here. A weak dwarf shouldn't be able to do it, a legendary maybe should be able to do this effectively?
Yeah, some function of creature strength vs. item weight should affect how fast he can swing it (for impact formulas) as well as how long it takes him to do it.
However, I'm not sure I see gold or lead ever being a good replacement for something like steel, for an actual warhammer. A warhammer cares about the shape that it's in, and softer metals are bad for that. It would matter less for a maul/sledgehammer, but I don't know how often those were actually used in war. Granted, DF DOES have them.
Really, I think it would be preferable to just make a larger weapon, perhaps with a bigger head, rather than change the material so drastically.