Thanks for the answers Christopher.
Regarding question 2, if I set prefetch to 200 MB, while streaming a large file, it won't show 200 MB consistently buffered, but rather it'll go to zero before it'll prefetch another 200 MB. Often it'll wait a minute before performing another prefetch. I don't know if this is a mere inconsistency between the GUI and the actual cached data.
Could the file be streamed too slowly for the prefetching to be triggered?