Im curious, does the underside of the board (bottom of picture) get warm at all? Looking at the design, I would think most air pressure will escape there, following the path of least resistance, and relatively little air will flow particularly over the top heatsinks. Id try channeling the air by blocking most or all of the clearance to the underside and using a fan with perhaps lower CFM but high static pressure.
Exactly, the construction reminds me of Avalon gen 1, you can narrow down the other channel to increase the air flow, but I think the effect will be minimal, since that original fan is a 2A high static pressure fan. The whole case is very cool, there is no direct contact between the heat sinks and the case
In my experience, the optimal output of 120x38mm fan is around 120CFM, where noises are still tolerable. Above that, you will dramatically increase the noise without too much improvement of airflow, a 200 CFM fan would sound 4x more noisy than a 120 CFM fan