I know that it doesn't ko but 252 CB Adamant Extremespeed Dragonite does 66.4% - 78.4% to min/min neutral Deoxys-S. Your Deoxys would be severely crippled just to take down 1 pokemon. You really wanna take that much just to take down dragonite? If so your team must be really Dragonite weak.
I'm not gonna bother arguing with you about BulkyDDnite because we're not gonna change our views on it. BulkyDDnite has the bulk to set up as much as it wants and therefore isnt outclassed by mence. That's my belief and weather or not you want to believe me is your choice. I wont respond to it either. However, I've been using BulkyDDnite sense before the set was actually suggested. I know quite a bit about it. Not saying you don't have experience with it I'm saying have have a lot of experience with it.
1) You have to predict Deo-S to come in, first off, and I don't see too many Dragonites going straight for Extremespeed first off, especially since there are next to no revenge killer Deo's running around outside the higher rungs of the ladder.
2)Bulky DDNite requires Lefties, Roost and in most cases a SpD investment. This means your damage output will be so much lower that you will need to take even more turns to set up. Here's how an average bulky DDNite set-up goes (assuming SR, which isn't realy too much of a stretch):
-Dnite switches in, takes SR (75%), possibly takes damage (if switching on something like CB Terakion, will take over 25%)
If you switched in on nothing -Roost while opp switches out or attacks (possibly 100% if you got a good blind-switch match-up)
If you switched in on an attack -Roost while opp switches (likely under 100% if attack did over 25%, which is likely)
-Now you are either at 100% or slightly less, and facing down a poke that threatens you. You now have at most 1 turn to set-up while Multi-scale is active, and when you are carrying Lefties and running a low investment in attack (maybe), your damage output will be very low.
On the other hand, you could use something like DD Mence, who takes just one turn to set up, is still not easily revenged, as he resists all priority but Bullet Punch, is far faster, and can switch in on physical threats to take advantage of Intimidate. Albeit he will be at most 75% assuming SR, but that allows him 8 attacks, even when running Life Orb, and he is able to take out Nattorei without Steel Thorns recoil. Now which seems like a better option?
3) I don't see how you using the set first is at all relevant, as I myself have tried it for a few days, which is more than enough to determine something's viability in most cases.