How fast would memory cards need to be to record 3k? Will CF work or does it have to be a SSD?
How fast? I'd be more concerned on how large the cards would have to be to hold just 5 minutes of footage at 3K. Then how much computer power you'd need to edit that footage.
Is it possible to make EF lenses continuously autofocus?
Unless you're shooting something moving fast it's really not necessary. Autofocus like on video cameras gets confused whenever something appears in the foreground and you're trying to focus on the object behind it.
Would the PL mount be negative to canon, because it will allow more third party lenses? Or does this make it more appealing for the higher end users?
I would just go with lenses like the Zeiss CP .2 primes with the EF mount. Far as I know the lenses made for the PL mount might be a problem for Full framed sensors.