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.