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).