Good question, I've not tried that. I'm sorry to say you will have to try it and judge for yourself. If I were to guess I'd say there is no benefit to using the slate as a layer and possibly a hindrance.
TT on Symposium Super Segue - slate or maple underneath?
I've gotten a Super Segue for my VPI Avenger. I have various things on hand I can place under the Symposium. I wasn't sure if a 3" thick maple block would work better than a 1/2" thick piece of slate. The whole thing will be sitting on top of my wooden equipment rack. I know ideally I should try both to see what sounds better, but hauling a 70+ pound TT on and off shelves for listening tests (not to mention moving a 3" thick maple block around) can be a bit of a pain. I'm hoping someone here has had some experience with what they've felt works best under a Segue, so I could try that first.