Found this on Tunacan whilst researching my 'new' 914 engine options--
When it comes to clutch discs, there isn't much choice. For the larger size (210mm-228mm) the selections are pretty limited to the stock discs. The only things to keep in mind is selecting a rigid vs. a sprung disc and the quality of the disc. The sprung disc features springs in the clutch disc that makes engagement smoother and relieves the drivetrain the shock of the abrupt engagement. The problem is that with "spirited" driving the springs can fail.
http://www.tunacan.net/t4/tech/clutch.htmsure looks like a 'rigid' disk to me
John