Yes, I agree that cone tracing is the most expensive part of vxgi (in fact, voxelization has miniscule cost compared to ). But for additional bounces, you can use less cones because the details are less noticeable:
For diffuse, 5 cones (1 normal, plus 4 in each tangent direction to a surface) would be sufficient and you only need a single cone trace for specular. In fact, for the first bounce, I’m pretty sure they only use a single specular cone which is treated as a ray in the direction of the reflection vector.
I have done in practice in my own experimental engine and it only at most doubles the cost of the cone tracing - was able to give me unlimited bounces. In fact, you can test the cost of these additional cones by adjusting the number of cones in the vxgi diffuse parameters.
I would be able to do in vxgi to demonstrate, but they need to provide some instruction that allows data to be read from the 3D texture, something like VXGI::LoadVoxelizationData (which currently doesn’t exist).