If you are at the point where you need to handle more IO, throwing additional memory onto a file origin will not in my experience do much because filesystems's

If you are at the point where you need to handle more IO, throwing additional memory onto a file origin will not in my experience do much because filesystems's idea of what to cache is vastly different from user's requests. Not doing any disk IO for hot files is vastly preferable than to doing some. This has been demonstrated in spades at LiveJournal which extensively stuffed files into memcache before Varnish became a thing.