Enough to keep crank case pressure down. Since the PCV is shut during boost the only place air can go is back to the turbo and intake pipe. As a test on my eclipse years ago, I put a vacuum gauge on the VC to confirm my setup worked, and it was always at least slightly in vacuum while driving. None of my seals (front main, rear main, camshaft seals) have never leaked.
Yup! It's nothing new, using a vacuum pump for crank case venting has been done for decades I believe. The more old school versions (usually built V8s) are belt driven, but an electric probably makes more sense in an application like ours. And again, this would just be the icing on the cake. Line/s ran to a vented catch can may be sufficient. I will likely end up testing this to be sure. If it doesn't then the 2 options remaining are running a catch can that runs back to the turbo to create the pull on the crank case (still getting a little vapor back to the engine) or a pump keeping it all external. This would only require 1 appropriately sized port/line. But, a second port with just a breather could also be of benefit as you would never pull TOO much vacuum on it if that's a thing.