Shadow elemental damage question

I'm tinkering around with a lightning arrow elemental bow build and i was curious if anyone knows if lightning damage% passives increase the damage of Lightning Arrow.

Yes,
But also +% projectile damage increase damage and far more since it increase all damage from skill (since skill generate projectiles) and not only lighting part of it. Also it gives +8%, and small nodes of lighting only +6%.
You will need to go to Templar for elem nodes cluster. Or to ranger / Duellist for projectile nodes. Better to Templar since you will need to take RT between templar and Marauder.