Tahm Kench: Devour minions from distance is reward, devour minions next to him is punishment

We talk about his mana costs. I tried it several times.

I devoured a minion from a distance, means using "Tongue Lash" (Q; 50 mana) and mid-air "Devour" (W; 90 mana). The costs remain at 50 mana. Without Tongue Lash, I take the full 90 mana costs for devouring. Means: It costs less mana when I use Devour safe from far away...? The only disadvantage I could imagine is his cooldown on his Q. But I think it's fine. The actual problem is when I use it on a minion next to me, e.g. when clearing a minion wave or Tongue-Devour just makes no sense.

In case I understood everything right and something is not fine with his W: Is it possible to do something like "If Tahm Kench devours a minion, half of his mana costs are refunded"? Of course without using the Q.