Wow 250W over 8", that will be a mondo bright system. Although I like how you've done it with the extra height so you can work in there and be able to look down on stuff.
I have a 24x24x12 frag tank (although I run it for carpet anemones not frags) and a single 175W using an Iwasaki 15k. I like the setup but I have the light only about 6" above the tank so I'm unable to look down on the tank. I definitely like your setup idea in this regard.
The one thing I've found with my tank - it is also rimless like yours (and to be honest, I don't think I'd do rimless again because of this problem) - but because I also have fish in the tank, there tends to be some slight splash effect and the water is constantly finding a way over the glass. I have to clean the outside all the time, and every few weeks I find a good little puddle underneath because it has wicked up one of the powerhead lines or something similar.
And another problem with having fish in such a shallow tank - far too easy for them to jump. So I have a piece of eggcrate sitting on top. It looks like butt.
I take it off for pictures but for the most part I find myself wishing I had a different shaped tank for my carpets and clowns.
But it would work GREAT for a frag tank. Yours will be a sweet little setup for you! Good luck! I can't wait to see what the flow is like with the Koralia's or the Tunze. I just use regular old-skool powerheads.. the prop style water movers would be better as long as they don't cause a wave to splash over the sides or something like that.