Hello,
Using an X70 with 4K license for the usual stuff; events, web videos, fundraisers, behind-the-scenes videos. Even though the 4K is 8 bit, and looks decent, is anyone using it's lower-rez, 10 bit modes and up-rezing it for cinematic/documentary work? Is anyone using an outboard SDI recorder with good results?

I can't imagine too many scalers that would upscale 1080 to 4k better than shooting in 4k. It's possible (some hardware scalers like Terenex) but if you've that requirement you might be better off shooting with an FS7 or equivalent that shoots 4k 10 bit 4:2:2.

Are you trying to save on storage requirements or do you find the higher bit depth is easier to work with later?

There is no way to get QFHD 4K out of the X70's 3G-SDI(B) connection to an external recorder as a technical limitation, but for HDMI there was an earlier discussion over whether the clean UHD output (with firmware version 3.0+) is 4:2:2 or 4:2:0 :

http://www.dvinfo.net/forum/sony-4k-ultra-hd-handhelds/531367-4k-over-hdmi.html

The X70 doesnt have SLog and only shoots in limited rec709-ish profiles. If you expose properly and want to do some light adjustments in post, then 8bit color depth is perfectly fine to handle that. I mean, you shouldn't need to be stretching very far for a rec709 delivery....you are capturing in rec709 then grading and delivering in rec709. Not a huge jump.

I'll take a rec709 4k 8bit image over a rec709 1080 image in 10bit ANY day.

Four times the frame size (UHD) is going to be FAR more noticeable and appreciated by common viewers than 1080p, 10bit color ever will be. In fact, 99.999% of people will be watching your work on 8bit monitors anyway. Now,...if you were shooting in SLog? Well, now the decision gets "a little" more complicated. ;-)

Yup, almost all Sony 4k cameras output 4:2:2 chroma sampling over HDMI.