I'm just looking for brand recommendations for pushing a few more CFM than my current 800rpm fans.
The best solution, given that you are restricted to a single 120mm exhaust fan by the Silencio 550 case, would I think be to replace the two supplied Cooler Master fans with high performance PWM fans. An example would be the Akasa Viper 120mm fan
from sources such as FrozenCPU. This is a fan with a 600 to 1900 rpm range. Probably the best way to deploy two fans would be one as exhaust and one as bottom front, retaining the existing Scythe fan at top front to provide hard drive cooling. These fans would be best used with a PWM splitter cable such as the Akasa AK-CB002
How these cables work is that that one connection is plugged into the CPU PWM fan header on the motherboard, and then the existing CPU fan and a maximum of two PWM case fans are plugged into the other connections. These connections are labelled on the AK-CB002. Power for all the fans on the cable is taken separately from a molex connector to the PSU. All the fans on the cable work at the same setting as the CPU fan. So at idle they all run at idle speed, but under the stress of gaming the case fans will increase their speed automatically. They are unlikely to go to maximum speed, instead you will find that there is a balancing point. Where that point is will depend on how hard the particular game is pushing the CPU and graphic cards. Because all the fans are being driven by the how high the CPU temperature is it will also depend on the ambient or background temperature in the room. So in a warm room in the winter it will be higher than a cooler room. As this balancing point is reached automatically, at least you don't have to try and predict it, which is the case with fixed speed 3 pin fans.
I appreciate it would require around a $40 spend for the Akasa Viper but it would improve the issue of insufficient case airflow. Yes, there would an increase in fan noise under gaming conditions. This would be balanced by the fact that the increased airflow would limit the CPU temperature and the noise produced by the CPU fan. Equally it would limit the graphics card temperatures and the noise produced by their fans. There may also be less fan noise from the PSU.