Objectively I would say that a benefit is that which has the greatest chance of increasing the health, life expectancy, security and safety, welfare, and/or prosperity in the greatest number of people for the greatest length of time.
Hmmm, I think I'm going to have to argue this piece by piece.
Normally I don't use this method to counter arguments, because it ignores how the pieces work together. So I shall address that issue at the end of my post.
HealthHow does one objectively define health? Physical health isn't too hard, as long as we have some sort of ideal human to aim for. I'm not aware such a specimen exists, however.
But mental health... now there is a problem. In the DSM-III, homosexuality was listed as a mental disease. So Homosexuality was NOT HEALTHY (according to the DSM-III). But in the DSM-IV, it was not listed as a disease. So it is no longer unhealthy. Now, this was probably a result of changing global attitudes towards homosexuality (as opposed to homosexuals changing their lifestyle). Therefore, the writers of the DSM-III used normative, not scientific, logic to make that decision.
Currently, there is a DSM-V in the works. The very fact that it needs to be made shows that there are problems with the current DSM. But this is still what people use to diagnose mental diseases.
Life ExpectancyActually, I'll ignore this one for now.
Security and SafetyOf what? Our health? That's already been covered in the health section. What about security of my freedom? What if I
want to do something dangerous because it pleases me? But this danger poses a threat to my health. Therefore, in order to safegaurd my freedom, you risk hurting my person. So then, does freedom outweigh it's risks? Obviously that would be a case by case issue, but how would you even go about measuring the pleasure derived from freedom in the first place?
Welfare This doesn't belong on your list, if you are using the basic definition of the word. All of the other things on your list fall under welfare.
ProsperityHere you run into your greatest problem. How to measure prosperity...
In many cultures, prosperity is defined by how many kids you have (and are able to feed). In addition to being a normative definition, this also defeats the purpose of population control.
So what about a strait up monetary definition. Afterall, money can be used to buy almost anything, and therefore is a decent representative of one's worth.
But then you run into the problem of Purchasing Power Parity (PPP)*. PPP dictates that money is worth what you are able to buy with said money.
Let me show you what I'm talking about. Say your system works out, and everyone gets rich. But the population control and eugenics program gets rid of people like Hemingway. And the only thing I want to buy is books written by Hemingway. So I'm not rich unless I can derive satisfaction from my wealth.
Therefore, the only way to measure prosperity by determining the total amount of potential satisfaction in the entire world's supply of goods combined with the individuals ability to acquire said goods.
But, as I've said before, there is no way to objectively measure the poetential satisfaction in a item. You can't just scan "The Sun Also Rises" through a computer to determine its worth. The pleasure I get from reading a book only exists in my own mind. This pleasure cannot be compared with anyone else's experiance, because no one can directly experiance another's emotions. So we only have our own feelings to relate this too.
This inability to measure feelings is the unifying problem with all of your aspects of benefits.
*That's not actually what PPP means. But it's a very important part of the overal concept, and the only part relevant to our discussion.