A good try, but personally I am not feeling it. Cannibalism might be shocking when it is a familiar good race doing it (i.e. Dark Sun's cannibal halflings), but as bad races go, it is pretty par for the course. Also, as was pointed out to me several times, skinny and dextrous has kind of been done as well.
(Edit: Also, in regards to humanity: yes, it could describe them, but if we stopped there we would not rectify the imbalance in D&D races.)
Why not just make them into the next step from Spartans?
I was just reading
this article
and thinking about your hypothesis. Around this I was thinking about two of the concepts: the Tasmanian Devil and the Shark.
The race reproduces at a rate much higher than would normally be expected by population norms... And there's a reason. The race matures physically quicker than it does mentally. The young are the size of a muscular ten year old by the time they are three and are used by their adult members as laborers for the next few years to supply their crops. Many individuals believe these laborers are actually a whole different race, as the population treats them as a slave race.
Juvenile: +2 Str. They have their Adult Strength, but all other skills are treated as children. They are frail beasts of burden, but they are used for their skills. They are Medium size by inches, but are useful in the field.
The young are sent into tunnels beneath the ancient cities to see who could survive. Years are spent in the tunnels, surviving off of lichen, detritus, and other children. The process separates the weakest and less intelligent.. And explains why there are never children seen among the race. Young Adults come out of the tunnels pale, wan, and hungry... They are rewarded with a life supplied by their younger brethren's toils as they are tutored and formed into their adult forms.
Adult Stats: As Pathfinder Humans.
What? Why would you do this? Because some will survive for reasons others would never understand. Intimidation, survival, an ability to disable the traps set around the complex/make traps, climbing, sneaking... There are plenty of reasons for such an individual to survive.
There are no reasons for extreme stat boosts in the population.
Slainte,
-Loonook.