The damage is actually far less than you think. Over the span of 30 seconds, his current W can be used twice for 40% magic damage at Lv1 vs 31% at Lv1 in the same amount of time. Factoring in 25% base magic reduction on everyone, that's 30% to 23.25% max health.
Now, take into account Tahm's mana pool, which is ~300 mana at Lv1 and ends up at 1005 at Lv18.
Currently, no one is going to let you get a free three stacks on them so you have to use Q minimum, which is 50 mana each devour. That's another 100 mana. He would be at 50 mana after factoring in mana regen, not being able to harass more afterwards, while the enemy is still at 50%-70% health.
And lastly, he doesn't max W first, otherwise he really would never get devours off against semi-competent players. Q > W > E so the damage is stagnant until Lv7/8, which is when he puts a second point in, and the third point at Lv10. Buy a merc threads, his damage falls off even more. Buy a spectral's cowl, well, Tahm's not going to kill you if you play it smart.
There's also the problem where Tahm would not be able to do ANYTHING until his W is back up so the enemy, who likely have sub 10 second CD as top lane or 10-14s CDs as supports, that's a 10-15 second window where Tahm basically is a cannon minion.