Years ago I had lead pipes replaced in the building where I lived. Zero difference... except for the fact that I'd actually drink tap water.
Anyway, growing up middle class in the 80s and 90s, we had a filtered water spout on our sink. If you weren't poor, they were fairly common. Once lead was removed from the supply lines, we still continued to use them since it also stripped out chlorine etc.
My point is, work-arounds for leaded drinking water weren't impactful enough to cause crime themselves.
If you want to attribute a drastic decline in violent crime to drinking glasses filling slightly faster you're going to need some extraordinary evidence.
Ahh, I see your point. Places with high amounts of lead also have high numbers of old buildings predating lead paint laws, industrial runoff and/or dumping, and many other sources--see Fruitvale.
Surprisingly, it's not really a huge deal.
EDIT: I don't mean to be a dick or even argue, I'm just pointing out that these problems do affect people, just maybe not most people through an effective combination of factors that reduces risk (where you live, whether you can afford a filter, etc). I certainly grew up on tap water and I am mildly disturbed people are ok with it causing massive mental health issues.
EDIT 2: Finally, I don't filter water I cook with; I imagine lead would still be a problem there. I probably should filter that water too in Oakland....