Hi Bruno,
Answer this;
Are you writing in C or assembly?
What is your clock speed?
Who's 8051 are you using?

I am currently doing a similar thing where I need to read data in 30us and have no problem doing it with a 11.0592mHz crystal running an 87C51FA Intel part and writing in C.

Determining your overhead is pretty easy once you answer the above questions.
Allen