Are deltafans and server cases the better way to go thermal wise? Because I could also just put the rigs in smaller stackable open air frames and enclose them in a custom built tent with a top exhaust cap connected to the vent. Fresh air could be just blown into the room, to use the cooled down air of the basement on hot days.
The key benefit is that you can shift some of the cooling requirements to the Delta fans, which are designed to run 200+CFM for ever, rather than GPU fans that will eventually fail (albeit after a lot of abuse). In practice this means you can throttle the GPU fans right back, moving heat away from the cards even more. This also means that if a GPU fan does fail, you have redundancy.
On the downside, they're louder than open frames, and you'll still be limited by the thermal capacity of your in-and-out fans, so you will get a denser setup, but not a bigger one (assuming you are currently maxed out).