If you plan to have a planted tank with CO2 injection then you will likely need
an additional circulation pump/wave maker, whether you choose the BioMaster 600 or 850. Any of these filters would provide enough filtration capacity, but might struggle with water circulation and CO2 distributions. The wave maker would provide the necessary circulation inside the tank. If you are not planning a planted tank, then go for the 850.
Not Oase's, but all vendors. You might get near the advertised flow rate in ideal conditions, i.e. without any media, very short pipes, and a near zero water column.
You can increase the flow of the BioMaster with a <
easy hack> in the pre-filter pipe (also check
this video at the ~10:00 mark).