...and additionally, how are Spec rebuilds handled? Do you get a rebuilt unit and then return your core to them? Or, do you have to ship your clutch and wait for rebuild and return shipping? If the latter, you have to live with significant downtime.
I'm skeptical of the benefits of "rebuildable" anything, unless it's for racing applications where frequent rebuilds are necessary.
I almost bit on rebuildable external reservoir shocks for my off-road truck. Then I heard complaints of extended downtime while waiting for the units to be rebuilt.
Cost/benefit of rebuild capability seems to depend how much the downtime matters and how often it is needed. For something as infrequent as clutch replacement, I'm not sure the cost savings is very meaningful. Particularly, when you have to trust that the rebuild service will still be available in another 9-10 years.
I think I just made the case for myself that the Stillen group buy is the way to go.