Alan B.wrote a review of Zipp SL Speed Carbon Stem on February 13, 2020
Familiarity: I've used it once or twice and have initial impressions
There's really not much more to it in person than you can see online. It's wider, a little more square, and for the true weight weenie, 13g grams lighter than the aluminum SL speed stem (comparing 110mm version of each). Photo shows the 110m carbon SL speed stem, weighing in at 127g versus 140 for the aluminum version (both are nice). I did not get out the calipers but side-by-side the stack height of the aluminum and carbon versions appears identical. The matte black finish matches the matte black version of Zipp bars and Zipp SL speed post. The visible layer of carbon fiber has a random orientation (neither unidirectional nor a weave). No stem cap (top cap) is included. It is a pricey stem, and no rational reason to upgrade other than you just want a matching carbon stem and are willing to pay the price.