With System 7, it is possible to set the cache size too high or too low. The default setting differs from Mac to Mac. If you are NOT running a CPU accelerator in your Mac, the default setting should be fine. If you are running a CPU accelerator, then that's where things get sticky. Sometimes the default setting will work well for Pro Tools, but maybe not so well for Sound Tools [we're currently revising SDII to ignore cache settings].
We have, on occasion, advised contradictory settings to users simply to keep disk-intensive operations from crashing.

Cache size not only affects the performance of our applications, it also affects the performance of the Mac. If there is a CPU accelerator of some sort in the Mac, that seems to effect Apple's 'recommended default', since they base that value on the machine ID, not the CPU speed. Comparing accelerated Mac cache settings to similar speed CPUs doesn't help, as the accelerators use instruction/data caching to get speed over and above the CPU clock speeds.