Checked the New PL/SQL SIMPLE_INTEGER data type, found following facts about it:

If value goes greater then 2,147,483,647 then it will became -2,147,483,648.
or
If value decreases less then -2,147,483,648 then it will became 2,147,483,647.

As a result without overhead of checking for nullness and overflow, SIMPLE_INTEGER provides significantly better performance than PLS_INTEGER because arithmetic operations on SIMPLE_INTEGER values are done directly in the hardware.