Tentative Assist Power's base power is 20 + 20 * each stage of stat boost, including the the doubling of boosts by Simple. Stat drops are not factored in. Highest power tested so far is 380; there may not be a cap.
Assist Power's increase in power will only acknowledge stat boost when said stats are at level +1 or above.
Stats drops are factored in, Assist Power's base power decrease by 20 for each stage of stat drop but will ignore stats that have dropped at level -1 or below.
The highest base power Assist Power can achieve is 860, to test this I used a Smeargle (103 SpAtk) and a Bronzong (364 SpDef).
Smeargle's moveset:
_Acupressure
_Tail Glow
_Spore
_Assist Power
Tail Glow first to immediately put Smeargle at +6 SpAtk in two turns, then Acupressure until Smeargle can't boost its stats anymore.
Then I used Assist Power against Bronzong and here are the damages:
194 - 180 - 200 -
176 - 200 - 178 - 184 -
202
Since there are 7 stats (including accuracy and evasion), max power is 20 + 20 * 42, which is equal to
860.
Stats drops are factored in, Assist Power's base power decrease by 20 for each stage of stat drop but will ignore stats that have dropped at level -1 or below.
Smeargle (103 SpAtk) vs Gardevoir (265 HP / 228 SpDef)
Code:
Smeargle: +6 SpAtk
Damage: 96 HP
Power: 140
After Gardevoir's Captivate
Code:
Smeargle: +4 SpAtk
Damage: 51 HP
Power: 100
Smeargle (103 SpAtk) vs lv 59 Magnezone (159 HP / 123 SpDef)
Code:
Smeargle: +6 Def
Damage: 45 HP
Power: 140
After Magnezone's Mirror Shot
Code:
Smeargle: +6 Def
-1 Acc
Damage: 47 HP
Power: 140
After Magnezone's Screech
Code:
Smeargle: +4 Def
-1 Acc
Damage: 34 HP
Power: 100
After Smeargle's Cotton Guard and Magnezone's Mirror Shot
Code:
Smeargle: +6 Def
-2 Acc
Damage: 44 HP
Power: 140
Assist Power's increase in power will only acknowledge stat boost when said stats are at level +1 or above.
Smeargle (103 SpAtk) vs lv 59 Magnezone (159 HP / 123 SpDef) second test
Code:
Smeargle: +3 SpAtk
Damage: 64 HP
Power: 80
After Magnezone's Screech
Code:
Smeargle: +3 SpAtk
-2 Def
Damage: 60 HP
Power: 80
After Smeargle's Cotton Guard and Magnezone's Screech
Code:
Smeargle: +3 SpAtk
-1 Def
Damage: 70 HP
Power: 80
After Smeargle's Cotton Guard
Code:
Smeargle: +3 SpAtk
+2 Def
Damage: 93 HP
Power: 120
EDIT:
http://www.smogon.com/forums/showpost.php?p=3071599&postcount=1225
It seems this was overlooked ^^'
Pumice Stone
Concerning the item Pumice Stone:
_it cuts in half its holder's weight;
_combined with the ability Light Metal, it effectively divides by 4 the Pokémon's original weight.
Pokémon used:
_Machamp
_Entei (198 kg / 436.5 lb)
_Rayquaza (206.5 kg / 455.2 lb)
_Metagross (550 kg / 1215.5 lb)
First test: Machamp (394 Atk) using Low Kick vs Entei (434 HP / 295 Def)
w/o Pumice Stone: 168 HP dealt (100 bp)
with Pumice Stone: 123 HP dealt (80 bp)
Second test (to see if Pumice Stone has a modifier /2 or /3): Machamp (394 Atk) using Low Kick vs Rayquaza (321 HP / 192 Def)
w/o Pumice Stone: 149 HP dealt (120 bp)
with Pumice Stone: 122 HP dealt (100 bp)
At this point we know the the Pumice Stone divide by 2 the weight of its holder, but what about Light Metal ?
Third test: Machamp (394 Atk) using Low Kick vs Metagross with the Light Metal ability (364 HP / 394 Def)
with Pumice Stone: 112 HP dealt (100 bp)
Now we know as well that it stacks with Light Metal as well (and presumably with Heavy Metal as well, but it's rather pointless).