nothing I have read about the plenum setup tells me it is better then just dumping the sand in the tank without a plenum.
as for mangroves, everything I have read says they are good, but slow growing, so nutrient export is low, and they are a little demanding in terms of care. most people use caulerpa in a refuge, but I have heard there are other good algaes to use, like gracilaria and sargassum. Dunno how to spell them, and I havent seen these algae's for sale anywhere, but there you go
and, I have no direct experience with the plenums or the mangroves - just making a statement based on the books I've read... maybe my reply gets this topic moving for you