Alot of what you said I can agree with, but I don't agree it's endemic to either African American or Hispanic cultures, even if there are specific weaknesses culturally that we can identify (such as broken homes).
But isolating the African American experience from issues such as poverty is going to be really tough, both because of our cultural history and the sample size to contrast it with is relatively tiny. Everyone still debates what the full causes are (whether that's poverty, institutional bias, or some combination), but I do think for many cultures there is something uniquely broken and I'm not against discussing the question. But it would be highly presumptuous of me to assume that I do have the answers, or that I'm some kind of White Savior who can rescue "those people" (I'm not implying that's anywhere near what you're saying, but just an admission of both seeing the issues, and not knowing the right way to solve them. But all cultures have a long list of social ills).

To answer your questions though,

> Is society better off from the averaging effect that comes from mixing high and low performers? The fact that performance is segregated racially is incidental to my point. Does a rising tide lift all boats?

Yes, a rising tide lifts all boats and sometimes drowns someone as well, but I think on the whole, even if the spoils are disproportionally distributed, our general societal success has raised quality of life for most people.

But your questions are fair, so I'll try to address the point.
An example I think about is with the Department of Education specifically.
I think we all agree that it's generally a good thing that we enforce kids school attendance, even if we hear upsetting statistics about a high illiteracy rate among high school graduates.
But what I do know is that even with that illiteracy level, on the average, more kids have a higher level of general literacy than many if not most of their historical counterparts that were kept home to tend the farm, and could get by without needing to read or write with any fluency.
Literacy seems like something that has degenerated in our society, but literacy was often a privilege of the wealthy, and I think it's averaged itself out better than if we were to replace it with nothing.

> Can we remain competitive as a nation if we improve our low performers at the expense of our best and brightest?

Honestly, can we be competitive without trying to help as many people as possible contribute to that success?
What would competitive as a nation even be considered if it were only limited to a small percentage?
Also, you're assuming that the cost to the "best and brightest" is one they can't shoulder.
By definition, they're more capable of addressing the issues than those who are merely caught in the cycle of perpetuating them.

Where I do agree with you is that education or a nicer school aren't panaceas to solving deep issues, but they do play a role (if they didn't, we wouldn't be having the discussion... if it's so important that the best and brightest need "good" schools, then I think we can agree that it would benefit not just them).
I also don't think this is something that just throwing money at is going to solve (it's actually played out where its the opposite now).

But IMO what you're describing is an amputation of sorts. And while we can believe that amputation is the proper remedy for certain cases, I don't think that should be the one we assume is the right one or one we have as the go-to, especially when there are many cases where things have been improved without that need.

I don't have a pet agenda to preach as the one right way to fix the issues that plague schools, but for good and bad, I believe we've generally been harmed when we isolate improperly, and generally benefited by being more inclusive, even if some of those kids choose to throw it away.
Alot of this will probably come down to how we feel comfortable with generalizing a lot of people and in reality the solution will probably be really sticky and take a ton of laborious and inadequately praised work.
But I say better that than replacing one flawed system with an even worse one, which I think is what you described (racially or economically segregating kids from access to an accepted level of education).

I disagree with the general premise, which is that the issues stem from inherent problems with certain minorites as well as that solving the issue somehow is addressed by segregation of those groups.
If I misread your point though, I'd be happy to learn where I'm missing it.