Just because it's serrated doesn't mean it should lose AP. I would say that if anything, the AP remains at base but instead Impact armor is 1.5x value against it. (in the interest of balance.), but adds the DV + 2. As for spikes it would make even less sense to raise the AP as those spikes would actually increase the AP. (focusing the kinetic energy of a blow into a spiked point should seriously increase the AP.)
You're misinterpreting it, a little. See, the reason I made the AP get worse when you put spikes on something, or serrations, is because the actual damaging part is spread out over a wider area, meaning that armor would have more opportunities to get in the way. Also, it's a balancing thing, 'cause putting some spikes on a club isn't really that hard to do.
If the weapon were getting a single, large spike, then yeah, give it better AP. This mod isn't focusing into a point, like you said, it's dividing it between half a dozen or more. So, it hurts more, but armor tends to work better against it. Against some wageslave in his oh-so-trendy business suit? That guy's going to wish he hadn't gotten in the way.
Regarding mercurial weapons. In all honesty, I threw this one in because I was starting to get a little strapped for ideas on how you could alter a melee weapon. At least, in a way that hadn't already been done to death. Technically, the liquid inside doesn't have to be mercury, especially in a world with nanotech and high-end chemistry -- what matters is that the liquid is dense, and heavy, and won't get gummed up.
To reflect how shifting weight can affect kinetic energy, try this little experiment. Take a 1-liter soda bottle, and hit something yielding with it (like, say, a loaf of bread). Make a mental note of what the bottle does to it, then drink about 2/3rds of the contents. Take another swing at it, this time letting the remaining liquid swing out to the end of the bottle. Odds are it'll hit harder, even though it's lost some mass.
There are modern examples of this sort of experimentation, mainly in the manufacture of golf clubs and baseball bats. They've also tried things like having a loose ball bearing trapped in the shaft.
So, any ideas on other possible mods? Maybe some things calling on SR tech that I haven't thought of?