I’m unconvinced of the need for a "chunky^ video cache.
We currently have around 190 MB of video cache. Ballpark figures: an 80 Mbit/sec average video stream is 10 Mbytes/sec. So we have 19 seconds of read-ahead cache to allow for peaks in the bitrate and/or network congestion. That ought to be enough for most situations.
The read-ahead cache is populated using spare bandwidth. Beyond 80-90 Mbits/sec the spare bandwidth becomes virtually non-existent and the cache is unlikely to be fully populated.
So the irony is that with high bitrate movies, when you really need it, the amount of read-ahead data in the cache might not be sufficient – and increasing the maximum size of the cache won’t overcome this basic limitation. For lower-bandwidth material, 190 MB should be more than enough.
That said, I think that there is possibly some scope improving the readfactor.