You know I'm actually not sure. Something you could test out though
Test Chamber: 5x5 area
Cobble roof with single dust layer above. 5x5x5 volume of witch water stopped by signs. Underneath signs water flows from 2 corners to single collection point. Hopper to ducts to chest item collection. 4 buckets water used total.
Conclusions...
Put sound off/very low or install Sound Muffler [this block is a godsend!]
Build this somewhat away from you [helps with spawning].
Witch water must be between y=62 and y=46 for spawning to work. Max efficiency would have the witch water start at y=62 and the signs at y=45.
If you're doing this w/o a grinder in the mix...suggest you build so there's 5 suffocating blocks above the trap [Ghasts are spawning/clipping through the ceiling and not dying until manually killed [needlegun works well here].
Very nice setup for tears. Now to get a blaze rod...hello angry doll.