I have Canon EF 2.8 lenses; 24-70 & 70-200. With a Metabones 4 smart adapter on the FS7, any reason to get the 28-135 kit lens? Would the 28-135 lens be sharper than the Canon lenses?

Depends what you shoot.
I've found the 28-135 very convenient because as a single lens it has a decent (almost 5:1) zoom ratio, holds focus, and does very nice smooth zooms.
I also have lenses in the focal lengths you mention, and when I want their 2.8 speed and shallower depth of field I use them.
But often for interviews I need something that works from 45 mm all the way to 120mm in order to reframe shots between answers. I can't do that with the still camera lenses, and I certainly can't switch lenses in the middle of an interview.
It's also reasonably light so going handheld is not a major stress the way a Cabrio would be.
Is it as sharp -- maybe not -- but I was really surprised how sharp it was and how well it compared after shooting a 4K job with the 28-135 as well as some high end stills glass.
It also works great on the A7s!
So my feeling is that it's a great value for the money.

Dave, I second that. If it only was wider at the "wide" end... Or something like Speedbuster existed to be put between a FF E-mount glass, and an S35 sensor camera! Or for that matter, if Sony made each lens in both APSC and FF versions :)

Well, we can dream...

Piotr

Thanks guys. Sorry, i've been away. Great responses. You told me what i needed to know. I do mostly interviews, some static, some with the camera tripod on tracks. Thanks again

I agree! My friend Doug Jensen turned me on to the Canon 200-400 zoom and, as usual, he was right on the money, beautiful performance. The Canon isn't inexpensive , until you price cinema lenses, then it seems like the deal of the century. I use my 70-200 Series II, my 100-400 Series II and my 24-70 Canon as well, all wonderfully crisp. The Sony, 28-135 is a very nice lens though, and pretty reasonably priced in the cinema world.