unless ive done something wrong in my testing; which is quite possible, it is not only unnecessary but will yield poorer results to not utilize mikktspace inside unity.
to summarize, for my own notes:
export fbx files after triangulation and only include smoothing group data. tangents and binormals are no longer needed, thanks to unity implementing the mikktspace tangent basis.
import into substance painter using the directX model and check the box to allow computing tangents per fragment. ensure all openGL/directX generators use directX as well.
export textures using the unity 5 preset, add in mixed ambient occlusion.
import into unity and under mesh settings for the fbx, import normals and calculate mikktspace tangents.
side note:
were mikktspace not integrated into unity (i.e. youre working with a version before 5.3beta5), it seems tangents should be calculated using the split model inside unity (marked as legacy in the newer builds). importing tangents to unity from fbx as suggested by allegorithmic seems to cause edge artifacts on my test model, even though i followed the pipeline to the letter (export tangents/binormals in fbx, and edit in substance painter without calculating them in the openGL model).
in addition, doing so seems to nearly double the vertex count, so having mikktspace available should cut down on mesh filesize and vertex count, as well as speed my workflow by allowing me to leave settings at default when importing/exporting between software packages!
sort of a headache to test all this out, but i think i finally have it sorted. going forward i should have a clear-cut pipepline to follow for 3d assets.