Yup, Mugen MBX7R front unis (MUGE2218) work on the front and rear of the D812 without any mods. You can still use the HB hexes. If you use the stock MBX7R hexes, your track width will be a total of 2mm shorter since each Mugen hex is 1mm shorter than the HB hex.
There are very subtle differences in traction between uni, cvd, and dog bones. HB already has a dog bone option which is supposed to give the most traction. I've tried the cvd and unis and can't tell the difference in traction between the two.
I usually use roll center, droop, or suspension settings to change the handling characteristics. Changing from uni to cvd would be such a subtle fine tuning option that most probably wouldn't notice the difference.
The big difference as many have noted is durability. I've had several cvd pins fail, so before any important race, I rebuild my cvds. It was starting to get expensive buying rebuild kits and new axles, so I eliminated all cvds and replaced with either unis or dog bones.