@ 1st paragraph:
The thing about Speed though is that while something with say, 394 Ak will always do x damage to y pokemon using z move, Speed is a very volatile (poor choice of words) stat. Between Sand Throw, Swift Swim, Choice Scarf, Thunder Wave, Stun Spore, etc., relative Speed can be changed drastically. I agree that Speed is a crucial stat, but I disagree that is the end-all be-all of stats. If it were, Deo-S would be banned and Roobushin would suck.
@ 2nd paragraph:
Hitmontop can only switch in on certain moves by Deo-N. Crobat could on,y switch in on certain moves by Skymin. Yes, it is not a counter. It is a check. But can we please not argue about semantics?And can we also avoid talking about the viability (or lack thereof) of Hitmontop? This is about Deo-N, let's keep it that way.
@ 3rd paragraph:
Basically, anything that it can't OHKO is a check to it. Because almost any decent power move (even things like Boil Over) can do massive damage. However, forget all of that for a moment. I'm not saying that Deo-N is OU material. But I am saying that the 15% drop in power (you said 15% right?) could possibly be just enough to make it not broken. I (and many others) simply don't know. What consequences are there if we test it and it ends up getting re-banned? What long-lasting effects would that have?