All physical specs of wood and hardware aside your weak points are as follows:
1. Speaker box construction (can it rack under its own weight, is it stoutly built enough to withstand point loading?)
2. Method of attachment of speaker to the wall (Is it hung like a typical wall cabinet, is it resting on a ledger...?)
3. Substrate supporting the load
4. Fastener type used
Personally, I would use the GRK family and disregard concerns over #s 3&4. What should be the determinant factor is can the speaker “hang”. You reference a 5’x4’ butcher block. To me, that suggests a large footprint, but small projection from the wall. If this is accurate, then the shear value of fasteners selected is what should drive the bus, because gravity is primarily pulling straight down. If the speaker(s) project from the wall, there are other factors to consider.
The points about sound transmission across and through the wall seem very valid to me, but I’m not a HiFi Guy. (Yes, I’m old...most do not use terms like HiFi anymore) [unsure]