Definitely room for compression improvement. For I-only I was able to get ~85Mbps: Canon 5D Mark III - I-Only compared to IPB
However, the extra data is mostly noise and artifacts. Doesn't look any sharper than IPB (~30Mbps for similar clip).
If anything, the IPB seems just a trace sharper.
Doesn't handle fine details in grass and so on like a C300.
I wonder if they can/will add a copped mode where they do 2x2 sampling like the C300 and maybe don't have to do as much AA softening?