Cool build, I look forward to pics!
To start off, I would ditch the carbon and replace it with more filter media. Carbon is only necessary when removing meds really.
You'll love the FX5, they're an awesome canister. Make sure you plug it in after hours though as every 24 hours it stops and lets any bubbles out, wouldn't want that every day during office hours!
I can't say I've used t5's before however I think at 800 deep you would need a few more tubes to grow carpet plants. Have you considered looking in to metal halide lighting?