I'm speaking a bit from experience and a bit of caution.....Again relying solely on siphon breaks can be disastrous...Another thing you want and should consider is type of skimmer you will be running.....12" water depth is way to deep for most skimmers to operate. Which in turns means you have to elevate the skimmer on a stand.....In turn this takes away any clearance you have underneath the stand to get the skimmer cup off......
I'm not saying by any means your design is flawed or wouldn't work, but if you look at melevsreef.com which I like to refer people to since Mark has done (does) excellent work, but you never see a sump with baffles that tall. Again not saying it can't or couldn't be done, but theoretically there isn't any need for it.....You always have to allow yourself extra headroom....Systems change over time.....
I understand what your saying about utilizing all space available, but I don't think your really gaining much if anything running a sump level that high.....Again I chose to run mine at 10" only after cutting them down, and that was more for the choice of being able to open my pump wide open if I choose or decide to opt to a bigger pump.
I'm curious you never mentioned size of DT or return pump as well in your original post......