I have exactly the same problem.
I want to distort a small shape path like a trapezoid. A simple perpective distortion.
So the pins of the corner pin effect has to be exactly on two corners of the shape. Thats only possible if i reduce the composition like you discribed. But if the shape is not parallel to the rectangular compostion window – that dont work.
I wonder if there is not a simpler solution. I tried other conrner pin effects like CC Power Pin and BCC Corner Pin but did´nt find a way.
Also there is a free transform tool in AE. But i can´t apply it to the shape path?