I think i have some results (even if i can't call them results)

I run a test with lvl 1 green sai and monitored his pure nin chase, with only him and water main vs 1 kakashi clones for 2 full 10 rounds, once with mei assist, second with gaara assist, water main using shark bomb every round to trigger his knockdown chase

with mei (all non crit) damage 149 - 146 - 135 - 137 - 147 - 131 - 130 - 128 - 132 - 130 (average = 136.5)

with gaara (all non crit) damage 138 - 133 - 142 - 141 - 136 - 130 - 132 - 121 - 136 - 130 (average = 133.9)

133.9 x 2% = 2.678 + 133.9 = 136.5

He didn't receive gaara 2% buff idk if same apply to hiruzen.

I think the system check ninja "external" attribute and doesn't bother looking "inside" ninja passives.

PS: sai have 5 elements just like hiruzen:

Yes, fire. My bad. I don't know why i tend to think is earth.