Here's what Milo (another "plumbing expert") had to say on these undermount sinks:

Milo said, "From my experience, we usually have good luck with removing under-mounted sink by inserting screw driver between the sink and front edge - and carefully applying pressure to it. You may even use hammer and gently tap the screwdriver few times. Don't try to break the seal the very first time. Do it gradually and every 6 inches or so all the way around. The joint will get weaker and weaker and eventually it will break off and release the sink.

Granite people usually use epoxy designed for gluing two pieces of stone together to attach the sink to granite. When this epoxy dries it has properties of glass. Meaning, it will break off relatively easy."... Milo.

You could give this a shot and see what happens. Before proceeding you may still want to consult with the silestone people... just to be sure... ;)

Mark