I would like to use a half inch thick foamcore board under each of my cake tiers to add height to my cake. Will it grip the point on the plate and keep the cake on the plate as well as the thin cardboard cake boards? Has anyone ever tried this?

lots of people do it. You'll be fine.

I haven't tried it but I don't see why not. Just make sure BEFORE you put the cake on the foam core that you press it onto the plate to mark the hole, then take a metal skewer or something and poke the hole futher into the foam core so it's a really good hole. I double and triple my cardboards sometimes and I've never had a problem as long as I pre-poke the hole.

Thanks for the quick replies.

and be sure to glue the cake to the foamcore with a good smear of bc