(I'm resurrecting the old thread, but I think it is still relevant).
I've been wondering about the same issue lately.
I can't remember exactly where the info came from, but there was a problem with too large disk cache size and recommendation back then was to use as small cache size as possible. It had something to do with UW drives on Adaptec cards and glitches in audio apps or something like that. The explanation was that SCSI drives already had large enough RAM cache and they didn't need to use the main RAM to buffer data for faster access. Since that time and, because I used SCSI drives exclusively on every Mac I owned (G4 included), I always set 128k as cache size.
Now, since switching to SSD, I've been thinking about modifying Memory CP to switch that cache completely off just to check if that brings any improvement. Anyone have any idea what resource should be changed in Memory CP to accomplish this?