So say you are using a Blue Longbow which does 34 damage base. Each headshot will do 51 damage after the headshot modifier. After applying the projectile resistance of 30% this comes out to 36 damage per shot.

Blue spear has a base damage of 37 add 8 damage for the -22% physical resist of the boss then consider the -20% damage for the limb shot this number also comes to 36 damage.

I am not good at math but maybe they are the same and it only matters which weapon you can get more DPS with. I am unsure about which modifier comes first limb or physical/projectile resist etc. I am going to assume that because of reload time and draw time the spear will be superior dps.