New FT-817 owner here with a SSB filter, considering getting a CW filter.
If I buy a 500Hz filter, would also using a SCAF-1 help for trying to pull out weaker CW signal, or would it work at cross purposes, adding ringing or a hollow sound?
What's your goal? DXing, contesting, or just ragchewing? There's a lot to be said for keeping a station simple. Fewer gizmos mean fewer RF and connection problems. Extreme selectivity could cause you to miss a station calling off frequency. Your brain can be a pretty good audio filter. Some contesters prefer wider filters. Plus It's not like the bands are that crowded nowadays.
Years ago I contested with a TS-850 years ago equipped with a Kenwood 270 Hz filter. I found that an outboard Timewave DSP-9, even in its 100 Hz bandwidth position, helped very little and I soon discontinued using it.
OTOH, I've discovered that the old Timewave was of immense benefit to my OHR-100A $150 kit radio that I sometimes use for QRP contesting. The Timewave is back on the operating desk.
To answer your question: The outboard filters I've used don't cause ringing or a hollow sound. But ultra sharp filters can add undesirable fuzziness to high speed CW because high speed CW is wider than slow speed. I own a Flex-5000 which allows the creation of custom brick wall filters. For CW I usually tune in a bandwidth of 250 Hz on the main DX bands during contests. Otherwise 500 Hz is fine.