Phil,
A more appropriate question would be how do you put 20kg extra into each hull apart from the extra 2 ft. and make sure all the extra weight contributes to increased strength and stiffness.
That has a long answer and to answer that fully you really need to compare the two sets of plans.
4mm ply was used because it was just what was needed to meet all the objectives of weight, strength and stiffness given the selected glass laminate.

The Blade F18 is very very stiff compared to the other F18s measured. Marcus has all the numbers as he peformed the comparative measurements.

Regards,
Phill

I know that the voices in my head aint real,
but they have some pretty good ideas.
There is no such thing as a quick fix and I've never had free lunch!