Re: "assuming the software is able to take advantage of the hardware"
Try to grade and encode 90/120min of RAW video on a tablet - if the software allows for it, and if you have enough storage.
IO will be the killer there but I can see the day when people do exactly that. USB 3 should be able to provide sufficient throughput. Course, the setup wouldn't really be that much different to an existing laptop-based solution. But that's the point.
But you can probably already shoot and edit 90 min of HD video on a mobile device. For editing you'll probably want a larger screen so a tablet makes sense but a phone would be able to do the encoding and decoding, probably need a power pack or better still electric connection.