Why don’t you have Torx head screws?

The Torx screw is characterized by a more visible head and creates more shading on the siding. It is therefore much less discreet than the Light Trim screw. With a thinner head, the bevelled undercut of the Torx screw creates pressure on the siding thus weakens the fiber cement panel.  

What is the advantage of using a grade 8 screw?

Using a grade 8 screw creates less pressure on the fiber cement panel. Therefore, reducing the risk of breakage. Since it is also smaller, for example than a grade 10 screw, it creates less fiber cement residue on your siding.