
The proof will be in the pudding when BMPC-4K ProRes HQ and compressed RAW sample footage becomes available. And the latter might be a ways out yet.
It's hard for me to imagine that BMPC-4K compressed RAW footage properly scaled-down to 1080p in post could be less-sharp looking than BMCC 2.5K uncompressed RAW properly scaled-down to 1080p in post.
Even if BMPC-4K compressed RAW footage loses a tiny amount of sharpness because it's compressed vs. uncompressed, my guess is because "4K" is so many more pixels than "2.5" the former may look at least as sharp as the latter after being scaled-down to 1080p in post.
Further, I wouldn't be surprised if BMPC-4K ProRes HQ footage, after down-scaling in post to 1080p, looks about as sharp as BMCC 2.5K uncompressed RAW down-scaled to 1080p in post.
Then there's BMPC-4K in-camera 1080p ProRes HQ footage; it may be a wild card. (1080p generated in-camera, not in post.) I suspect it'll look sharper than BMCC in-camera 1080p ProRes HQ/DNxHD footage, but that's just another guess.
Will be interesting to see!