Mary
First not sure we are talking about spinning Gliders/powered or flying gliders above VNE at FL300.
Firstly as a non Glider pilot I am surprised that they fly up at FL300! most I have seen are below cloud base ie 3000-5000 feet. a couple of times above over the Alps one being around 13000 feet.
Am amazed with the claims that some are up around FL300??? and braking VNE at TAS rather than IAS?
Having said all that I think gliding would suit me so maybe I should have a go?
Pace