HDD cache is only useful for that data that is most frequently used, or re-used in quick succession (that is, before it has been overwritten). New data typically overwrites the oldest data.
The OS will cache data it considers important in reserved sections of RAM. For example, Win buffers the data to be written to the page file, so that there are not a whole lot of little writes that would bring a HDD to its knees. Drives cannot know what is important to the OS, and the OS cannot tell it either. Basically, the OS tells the drive where its wants the data, and the drive just treats it all the same way, using its cache even-handedly.
If the amount of new data to read off the drive is continually greater than the cache (no chance for the old data to be re-used), then the cache is providing no throughput benefit, but just buffering the data, which may even out the flow depending upon its size.
Basically, for large heavily-used libraries, larger cache will probably not provide much benefit as the multiple simultaneous streams will almost always be new data (next block of each playing note), and swamp any cache that is not hundreds of MBs. If you are really pushing a HDD that much you may be better off using an SSD.