Thank you all for your replies!
I have all of the various Warbirdsim P-51D/K products installed in P3D, and there are just a few items that stand out to me as being the result of them being port-overs:
1- The self-shadowing on the dark side of the fuselage facing away from the sunlight tends to add some shadows of parts that are on the interior model (which I've noticed with other FSX planes ported to P3D V4 as well). I think the only way to fix this would be to recompile the models as P3D-native.
2- For some reason, the way I had the K-14 gunsight reticle system setup in FSX doesn't work properly in P3D V4 (there is something different there about the way P3D V4 handles the visibility parts/code). What happens is that the reticle is always on no matter what (and you can't change between the two different reticle types either, as what worked in FSX). As cool as it looks, I never fly with the gunsight turned on, so as the most simple/easy fix, I just blackened-out the alpha channel of the reticle texture(s) to permanently disconnect the power to the gunsight.
3- There is a weird artifact with the white navigation light effect for the tail (as I also saw in some of Jan Kees' screenshots) - I have redone the effect file to fix this problem.
4- The radios won't function properly until making a small little edit of the Aircraft.cfg file.
Items to address numbers 2-4 will be included with all repaints I release from here on out.
For this new repaint of "Sierra Sue II", I currently have three different versions completed (to upload together as one), so that the repaint can be installed into either the "Little Friends (I)" product, the "Happy Jack's Go Buggy: Then and Now" product, or the "Mustang Tales" product, all with unique/total compatibility, all with new exterior textures with new HD bump/normal maps, and with new/modified interior textures as well. No matter which product you choose, the repaint will look 99% identical, but with some minor differences to address some of the different model configurations. There are also two different options with this repaint that you can choose between to install (for any of the three products) - one that is more freshly restored, with a light polishing, less weathering and rainbowed exhaust stains - and the other being a slightly more aged appearance (following the past three years of operation), with more patina overall, with heavier lead-filled exhaust stains and standard oil staining. Both types can be compared in my screenshots posted above.
If you don't own any of those three and there is another particular Warbirdsim D-model product you own that you would like me to make a version of this repaint for, please let me know.