I'm sure that, given their lower max health, small camps will go slower early/midgame (believe it evens out at like 90 dps vs the giant wolf at fiveish minutes, that's easily doable, even as tank). However, consider the second blue buff:
1900ish health. 32 damage per second from a quill coat. You'd need 128 damage per second for a very extended period of time itemless for spirit stone to be superior. Autoattacks won't help too much, he has 20 armor and you have less than one attack per second. Seems like quill coat kinda wins here, somewhat?
Then, sustain, to beat a quill coat you'd need to deal 667 damage per five seconds (6% is not a lot of health, 40 is). 1000 for mana sustain, that's 200 per second as a tank. Quill coat is superior in sustain.
So the quill coat appears to be made to speed up buffs a bit depending on champion at the cost of small camps in early/midgame, give them a ward/20 armor and to insanely buff their jungle sustain. I see no issues with this.