I would never run ONE Fx5 on a 350 gallon tank alone. I would use atleast 2 Fx5s.
First, assuming bio load would be great and your turn over would be less than twice with a 600 GPH flow rate.
Two Fx5s you would have just over 3 cycles per hour. Not sure if that woudl be enough but you would have 1200 GPH. and 10 liters of bio media.
If you modded the Fx5s you would get about 1200 GPH with 18-20 liters of bio media.