I tried a ceramic sponge filter several years ago, and while the seller assured me that it would easily pass iron, it did not.
The problem was that at the time, I did not know how to correctly tune an oil burner, and so my iron was not hot enough to pass through a filter.
I was really surprised that 100model's filter passed the iron with such a slow pour rate, so that speaks volumes about how good his pour temeratures are.
The filters I have seen in commercial use typically have a sprue that is about the same diameter as the filter, which perhaps 2.5" diameter, and generally round.
The filter appears to also function as a velocity control, and the parts in the flask can be radial around the large spure, with no runners at all.
So you just pour the sprue full as fast as possible, and hold it full.
A slow pour is generally going to clog the filter I think, or has the potential to clog the filter.
The gates could be oversized if the filter is acting as a velocity controller.
Filters are used extensively in commercial foundries, but they have induction melters, and can easily add quite a bit of superheat to the iron.
I think with an oil burner, about the best maximum pour temperature that can be reached is in the 2,500 F range, perhaps 2,600 F, but that is a guess judging from what others have said.
Here are the sponges I purchased, and the spue that I used the filter with.
The last photo was from another pour with a filter, that also came up short.
At the time, I was pretty clueless about foundry work, and just blindly stumbling through the casting process.
I think I could use filters now successfully, but I use spin traps at the end of the runners, and they work as well as a filter, and don't clog if the pour temperature is on the low side.