First let me say that this UI could and probably would make me STOP using Delta3.0 entirely... Always found it a bit too dark for my liking... And this is a VERY clean UI..
That being said... Im looking at this from a high-end, time-enabled point of view as well...
Use a view port... or at least it LOOKS as though your designing this to be used at set resolutions... Everything I see here looks simply brilliant but I DO see a problem with chat box positioning... The only real way I see to get around it is to raise the bottom portion of the UI (Hotkeys, group, target info) and place the chat windows underneat... Otherwise your going to have entirely too much of the viewing area being taken up by awkwardly placed chat boxes... Using Delta3.0 right now and 4 chat boxes (2 dedicated to melee, 1 to raid channels, 1 to guild/group/say not to mention emotes, other info etc) it all kind of builds up
Silding the chat boxes underneath would let you dedicate 2 to melee or spells and 2 to chat (guessing roughly from the size of items displayed)
That being said...
It looks simply amazing...