I'm not suggesting a Nerf is warranted. Cause I think IDK need to stay at around their current win rate. But they aren't strictly mid-tier right now.
Since the December FAQ, IDK have the 3rd highest win rate (out of 28 factions). It's an important stat, but it is a relatively small sample size. So if we take the whole of 3rd edition instead, they move to 7th out of 28. 3% behind the LRL Win Rate.
What IDK really need (and I'm not sure if GW are clever enough to do it), is to be less oppressive vs. the lower tier armies, but do better vs. the top 5-6 armies. Currently IDK absolutely DOMINATE the lower tier armies, but they have a sub-par WR vs. the higher tier pack. If they could be re-balanced to compete vs. the top end, but not be so disgusting to the bottom end, that would be ideal.
Essentially, IDK is a faction that *could* afford to drop in power and would still be considered solid. I wouldn't want to see that though, it's just worth bearing in mind, that they aren't in the kind of position that Fyreslayers, or Ogor Mawtribes are ... where a reduction in power would take them tumbling into the very bottom of the pile.