WD projected that 8tb drives might be ready in limited quantities sometime btw Q1-Q3 of next year.. not sure about toshiba and any other MFG's left..
I don't think the rpm has much to do with the speed these days.. 5400 rpm drives of today have a bit more speed than 5400 rpm drives of the past.. also, I've seen variations of rpm such as 5800rpm - 6000 rpm. Perhaps there might be variable rpm coming soon vs a fixed rpm.
The new 4k codec HEVC promises much higher quality in a smaller density footprint (225 MB per hour of video?!? @ 4k resolutions & PRO HD multi-track sound+ subtitles!!) You might look into those. But be warned, these files are NOT apparently compatible with most tablets b/c the apps which are being created (to playback h.265 hevc) are apparently glitchy or don't work well -- yet. I bet they're going to hock the next generation of tablets as "ENABLING" this hd codec..slimy marketing..