G10 is layers of fiberglass in an epoxy matrix and yes, if available I would have used it. But it is both massive overkill to the design requirement, in addition to not having been available when I needed it. If thinking about cost (I wasn't) it is also very expensive.
Bluewater Coosa is a high density fiberglass/foam product that is made to be a direct replacement for plywood. It's a graphite polyurethane with both layers of woven fiberglass and continuous (loose) fibers so it is hard as a rock, which is why the large yachts use it for their through hull backing plates, in addition to it never rotting and being less expensive than G10.
As for compressive strength, remember that the boat designer determined that the strength of 1/2" plywood was sufficient, so anything stronger than that would be an improvement. The compressive strength of marine grade plywood (6,400 psi) is about 1/10 that of G10 which is why I said the G10 is massive overkill.
The 3/4" Bluewater Coosa I used is 11,810 psi so is approaching double what the original manufacturer installed and I'm fine with that. It will also never soften or rot to look like what came out.
On a tangent and from an engineering point, backing boards are really no longer needed with the integrated thru-hulls from Perko, Forespar, and others. The point of the backer is to create a wider and taller base to spread the load so the valve would not break at the threads rising above the top of the thru-hull nut when a 500 pound load is applied to the tail/top.
The integrated thru-hulls have a wider base which already distributes the load enough to pass the 500 pound load requirement of ABYC H27.
