Yeah, the double layered foam does tend to get heavier than you'd expect. I still like double layering the foam for aerodynamic and durability purposes, but if you need to move the CG forward, eliminating the second (usually not necessary) layer of foam in the tail does help.
Aside from that, The build looks promising. I was going to do the SS as I love biplanes, but Chief Oshkosh has the local history value, and I'm helping to restore the plane that Chief was converted into (Buster.)