Yeah. Ideally the EFW would have been designed with the filters as close to the camera as possible within the EFW. It's hard to tell but they might look closer to the OTA side than the camera side, so f/4.5 is a decent guess. If they were in the middle of the 20mm wheel they'd vignette at f/4. If the EFW could be reversed with an M63 opening on the OTA side then it might be close to f/3.
Any of those options, f/4.5, f/4, or f/3, are still much better than limiting themselves to f/5 due to m54 threads.
Edited by chadrian84, 18 November 2019 - 06:02 PM.