Plywood has limited strength since half the layers are not doing much.

It is not the material "support it own weight"
but supporting the load of a person leaning down on the furthest edge of the material.

Corrugated cardboard can support it own weight, care to lean on it?

I'm not familiar with silestone, so that's why I'm asking. I don't have a feel about it's strength and if it was in the specs, I couldn't cull it out.

I'm sure that people do cantilever it for counters.

Post on the kitchen forum. They will know.

Assuming you aren't wanting brackets, there are ways of "letting in" support rods or flat metal runners to give support without being seen or interferring with your kness. Im not understanding why you aren't addressing this with your counter people. If they are reputable and experienced, they will know how to address this. If you are going the big box store route, maybe not.

Towards the end of this kitchen forum link are some guidelines, but yes, your installer should know--make them do it and warranty it!

(Pretend Silestone is stone for these purposes.)

Here is a link that might be useful: kitchen forum stone link