Here is a video that gives you an idea of how smaller vent covers, including the MaxxAir II, affect air flow on a Fantastic Fan. The MaxxAir II is demonstrated at 1:12 in the video.
Camco's website for the MaxxAir II says, "Can be safely installed over high powered RV ventilator fans; however our Fan/Mate® is specifically designed for this application allowing higher air flow from your existing ventilator fan." So, yes it can be used over higher powered fans like the Fantastic Fan but it's not the best option.
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