Great to get information from others.

The step-down buck converter uses the LM2596, it has an output rating of 3 A. The electromechanical specification of the PCIe bus states that the maximum burst current at 3.3 V should be 2700 mA and max nominal current around 2000 mA. The intention was to have one step down converter per riser for modularity and the step-down converter is less than a dollar per unit.

http://www.aliexpress.com/item/10-PCS-DC-DC-Step-Down-Converter-Module-LM2596-DC-4-0-40-to-1-3/1975111111.html?spm=2114.01010208.3.2.yAEyBa&ws_ab_test=searchweb201556_0,searchweb201602_1_10037_10017_407_10033_406_10032_10040,searchweb201603_8&btsid=3513d8de-8fc2-4050-b7dd-6f5295aa688f

I like the idea of having one big converter to power all risers.