Vignetting, if due to set aperture, should be worse wide open, than at F11. I often see a slight very slight upper corner vignetting on the P4 both upper corners.
Color cast, is most definitely possible, the Mavic has this also, worse than the P4. Easy way to test is just shoot against a pure white background. It's also possible to create a Lens Cast Correction in software like Capture one that will correct all of this. However Capture One has other issues with the raws from the DJI drones as no default color profile is loaded.
LR/ACR are using an embeded color profile, which I don't like, but unlike Adobe's other camera profiles you really don't have many other options, even with the latest version of LR with the new profile grid, there is not a lot that helps. LR however does seem to load a lens profile for the various Phantoms that can help take out some of the cast, and vignetting.
Shooting in raw IMO is best as you still have a lot of control of the final image. And I shoot most series in AEB mode, 5 shots even though I know I will throw most at least 2 from each series out. Due to limited air time and the fact that the images when displayed on the iPad are not very good quality (you can't really tell the true color and focus) I just prefer to over shoot knowing that from -1.34 to +1.34 in 5 shots I will get 2 I can bracket later on.
I agree that the sweet spot seems to be F4.5 to F6.3. I have gone to F8, but feel that much smaller a setting will just suffer from diffraction. I also tend to stay in 4:3 since I shoot a lot of panos. I realize you are giving up a bit of resolution, but the 3:2 shows quite a bit of rectilinear distortion which is common with any wide angle lens. (objects towards the edges will elongate and flatten), and this makes a pano creation a bit harder. I would prefer to have the landscape orientation option that the Mavic gives, but on the P4 you might just get too much props showing.
Paul Caldwell