So you really believe that there are no inborn differences between humans at all?
Nice idea, but unfortunately the evidence indicates somewhere between 1/2 and 3/4 of human intelligence is determined before birth. It can be degraded but not increased, as with so many human characteristics.
That some people exceed their own and others expectations does not address this issue at all. It just means that the expectations were mistaken.
There are differences to be sure. Retarded and autistic people are an example. But, with hard work anyone can improve and who is to say what a person can achieve if they don’t give up.
I do not believe there is some absolute limit to learning. It is an ongoing process that by it’s definition extends a persons potential as he learns.