I calculated out some probabilities based on a single turn of encountering an opposing Drifloon in a dungeon room, assuming you cannot disable Unburden and it uses Ominous Wind at every opportunity.
The list below is absurdly long and covers all 140 different scenarios, but I've condensed them into spoilers.
Assumed mechanics:
- Unburden allows the Pokemon to make two moves in one turn, using only 1 PP.
- Ominous Wind has a 10% chance of boosting the user's Attack, Sp. Atk, Defense, Sp. Def, and movement speed by 1 stage.
- Stat boosts cap at +6. Movement Speed caps at +3 (4 moves per turn).
- At maximum speed, a Pokemon with Unburden can make 8 moves in 1 turn, using 4 PP in the process.
Key:
N = No boost
B = Boost
X = Boost not applicable because all stats are at +6 already
In the list below, something like "NBBNNN 570" means:
- Drifloon makes a move. Ominous Wind is used twice thanks to Unburden. The second time gets the 10% chance of a boost. (NB)
- Because Movement Speed is now +1, Drifloon can make two moves this turn, so it gets another move. It uses Ominous Wind twice thanks to Unburden. The first time gets the 10% chance of a boost. (BN)
- Because Movement Speed is now +2, Drifloon can make three moves this turn, so it gets another move. It uses Ominous Wind twice thanks to Unburden. Neither time gets the boost. (NN)
- Drifloon has now made the maximum three (six) moves for its Movement Speed, so the turn ends.
- Overall, the move did 60+60+90+120+120+120 = 570 base power damage, plus STAB. Ominous Wind now has 2/5 PP remaining.
And now for the gigantic list:
No boosts:
Probability 0.9*0.9 = 81%
Average base power = 120
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One boost:
Probability 2*0.9^3*0.1 = 14.58%
Average base power = 315
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Two boosts:
Probability 5*0.9^4*0.1^2 = 3.2805%
Average base power = 592.5
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NBNBNN 540
NBBNNN 570
BNNBNN 570
BNBNNN 600
BBNNNN 630
Three boosts:
Probability 14*0.9^5*0.1^3 = 0.826686%
Average base power = ~919.3 (12870/14)
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NBNBNBNN 840
NBNBBNNN 870
NBBNNBNN 870
NBBNBNNN 900
BNNBNBNN 870
BNNBBNNN 900
BNBNNBNN 900
BNBNBNNN 930
NBBBNNNN 930
BNBBNNNN 960
BBNNNBNN 930
BBNNBNNN 960
BBNBNNNN 990
BBBNNNNN 1020
Four boosts:
Probability 43*0.9^4*0.1^4 = 0.282123%
Average base power = ~967.0 (41580/43)
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NBNBNBNB 840
NBNBNBBN 870
NBNBBNNB 870
NBNBBNBN 900
NBBNNBNB 900
NBBNNBBN 930
NBBNBNNB 930
NBBNBNBN 960
BNNBNBNB 870
BNNBNBBN 900
BNNBBNNB 900
BNNBBNBN 930
BNBNNBNB 900
BNBNNBBN 930
BNBNBNNB 930
BNBNBNBN 960
NBNBBBNN 930
NBBNBBNN 960
BNNBBBNN 960
BNBNBBNN 990
NBBBNNNB 930
NBBBNNBN 960
BNBBNNNB 960
BNBBNNBN 990
NBBBNBNN 990
NBBBBNNN 1020
BNBBNBNN 1020
BNBBBNNN 1050
BBNNNBNB 930
BBNNNBBN 960
BBNNBNNB 960
BBNNBNBN 990
BBNBNNNB 990
BBNBNNBN 1020
BBBNNNNB 1020
BBBNNNBN 1050
BBNBNBNN 1050
BBNBBNNN 1080
BBBNNBNN 1080
BBBNBNNN 1110
BBNNNNBB 900
BBNNBBNN 1020
BBBBNNNN 1140
Five boosts:
Probability 48*0.9^3*0.1^5 = 0.034992%
Average base power = 1035
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NBNBNBBB 870
NBNBBNBB 900
NBBNNBBB 900
NBBNBNBB 930
BNNBNBBB 960
BNNBBNBB 990
BNBNNBBB 990
BNBNBNBB 1020
NBNBBBNB 930
NBNBBBBN 960
NBBNBBNB 960
NBBNBBBN 990
BNNBBBNB 960
BNNBBBBN 990
BNBNBBNB 990
BNBNBBBN 1020
NBBBNBNB 990
NBBBNBBN 1020
NBBBBNNB 1020
NBBBBNBN 1050
BNBBNBNB 1020
BNBBNBBN 1050
BNBBBNNB 1050
BNBBBNBN 1080
BBNBNBNB 1050
BBNBNBBN 1080
BBNBBNNB 1080
BBNBBNBN 1110
BBBNNBNB 1080
BBBNNBBN 1110
BBBNBNNB 1110
BBBNBNBN 1140
NBBBNNBB 960
BNBBNNBB 990
NBBBBBNN 1080
BNBBBBNN 1110
BBNNNBBB 960
BBNNBNBB 990
BBNBNNBB 1020
BBBNNNBB 1050
BBNNBBNB 1020
BBNNBBBN 1050
BBNBBBNN 1140
BBBNBBNN 1170
BBBBNNNB 1140
BBBBNNBN 1170
BBBBNBNN 1200
BBBBBNNN 1230
Six boosts:
Probability 20*0.9^2*0.1^6+6*0.9*0.1^6+0.1^6 = 0.00226%
Average base power = ~1116.7 (30150/27)
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NBNBBBBB 960
NBBNBBBB 990
BNNBBBBB 990
BNBNBBBB 1020
NBBBNBBB 1020
NBBBBNBB 1050
BNBBNBBB 1050
BNBBBNBB 1080
NBBBBBNB 1080
NBBBBBBX 1110
BNBBBBNB 1110
BNBBBBBX 1140
BBNBNBBB 1080
BBNBBNBB 1110
BBBNNBBB 1110
BBBNBNBB 1140
BBNBBBNB 1140
BBNBBBBX 1170
BBBNBBNB 1170
BBBNBBBX 1200
BBBBNBNB 1200
BBBBNBBX 1230
BBBBBNNB 1230
BBBBBNBX 1260
BBNNBBBB 1050
BBBBNNBB 1170
BBBBBBXX 1290
I hope I didn't make any mistakes in figuring the above out. If the above is accurate, then in the given scenario, Drifloon spamming Ominous Wind will do an average of 173.3 Base Power damage on the first turn, plus STAB. However, note that the 'mere' 120 base power for the 81% chance of getting no boosts drags this average down significantly. It would probably be better to represent the damage output as:
- 81% chance of 120 Base Power damage, plus STAB
- 19% chance of an average of 400.4 Base Power damage, plus STAB
Essentially, you're getting hit with something like STAB Outrage no matter what. However, on a chance approximately equal to Stone Edge's miss chance, you're getting hit with something on the order of three consecutive Draco Meteors all at once, without the stat drops. And with STAB. On the very first turn. That is going to hurt. A lot.
Also note again that this is only the first turn. Since it is hitting from across the entire room, it is likely to be able to attack again, and this time the boosts apply to all the hits, not just the later ones. I'll work out second turn probabilities later, if I feel like it.