When looking at chassis to support >15 drives, I am just window shopping as I have no need for that amount of space.
However I came across this case and thought some might be interested. I really like it although it isn't a full x-in-y solution (9 internal bays, up to 13 external bays using a pair of 5-in-3's and one 3-in-2).
http://www.nexustek.nl/NXS-Nexus_EDGE_full_tower_case_with_noise_absorption_foam.htm