I know exactly what Chris is talking about. If you lift up burried rock it is totally black and grey and stinks like sulpher. I can't see this being a good thing. Although im not sure if it will cause algae blooms.
In hindsight I had this byropsis type algae on my return pipes and a bit on my overflow when I had burried rock.. hmm. But reciently i re-did my whole rock work (placing the rock above the sand this time) while removing my dreaded grape caulerpa and now it is going away....I have no clue because around this time I got new MH bulbs, new skimmer, and upped my kalk dosing. All factors that would decrease problematic algae.
anyways, about your tank (i got a little side tracked lol).. it looks really good but having a wall of rock up against glass would scare me personally. you can cheat and make it hollow behind there. some sort of flow behind there will do good in the long run. putting powerheads behind and under rocks is ok... untill you have to clean them.
i dont think 900gph out of one return is to much either as long as it isnt blasting corals. maybe idealy you want to split it into two 450pgh returns at each end. as your tank gets older you will find what works best for it.
keep up the nice work
