Hey FMJ,
I've been thinking about your prospective third cab since you first proposed the idea a while ago.
First, let me say that I think you've done a 1st rate job on the project so far.
I'm not sure where you are planning to put this monster, but 9ft of metal cabinets is a bit conspicuous. Especially if they seems to have wind whistling through them and humming machinery inside.
Aside from that, I think it would be pretty kick ass to have a internally cooled, self-contained cab! I'm not sure what the water cooler is all about though. Wouldn't the AC act to dehumidify as well when it is running. Also, the AC would have to be in a section with separate ventilation or it will heat things up.
If the room's ambient temp is contributing to the high internal temps, maybe the addition of an evaporative cooler or portable AC outside the cab directed at the intakes would be helpful.
Maybe it will just take some experimentation on the controller to reach a balance between necessary cabinet ventilation (and heat control) and CO2 administration. I would lean toward increased ventilation over CO2 as I have seen some amazing buds grown without the need for supplemental CO2.
Also, if I remember correctly from earlier in the thread, you are planning a cross-country move in the relatively near future. Maybe you could see how things are working with the current set-up and add the third cab. after the move if you find it necessary. Hell, you may find the perfect house/apartment and decide to set up a grow
room. I'm not sure why, but I guess I am discouraging you from the third cab just now... I just think it is a bit overkill.
I truly wish you the best of luck and look forward to seeing how it all works out!
~Rock~