I think I basically ran my BoF's the same way as posted directly above, and I think as intended - if it's not done as an advantage, there's no mechanical reason to impose a penalty - if your stunt that has no other appreciable effect, DOES have the possibility of an effect, such as impressing an NPC (and that impression being important to the game in some way), then I consider it a roll, otherwise - no.
When I run games, I more or less used Queex's method, though before I had read it so not nearly as refined. Once a roll exceeded the 5 outcome for one mook, for every... I think I did 3, points above, I considered it another mook taken out, and the players certainly never complained, and it did help them hesitantly narrate broader attacks from their characters. I guess really you could just set a "factor" or something for each type of mook group - say standard mooks get taken out on a 2-1 basis (Queex's divide the remaining outcome by 2), while the way I did it, it would take 3 more outcome per extra mook wiped out, etc.
The "downside" (which I don't really consider it) is the multiple enemy penalty only really gets used in a particular situation like a hostage or with named characters. The upside is of course, bare-assed awesomeness for the characters and quicker, easier GM'ing, also not a bad bonus for me.
Obviously, the multiple trashes mechanic works best in conjunction with something like the true mook rule of ambiguous numbers in a group - instead of 7 mooks, it's "a handful". It's not definite, but when was the last time you or the heroes stopped to count the mooks in an action movie before sticking a foot through someone's esophagus? "How many are there?" "A shitload!" Since mooks don't take damage anyway, I use the borrowed Threat Level (calling it Life as it's more a question of how much/long it takes to get rid of them in a scene) idea to give the whole enemy group for the scene a "Hit Point Pool" essentially, and whenever it runs out, the fight is over - either more mooks pour in, or downed ones get back in the fight, etc. until then. The number of mooks that attack each turn would I guess logically be the same as current Threat Level (calling it divided by Guts, which could vaguely equate to the base number of mooks you could specify if you were pressed.
You could use the extra mook reduction division factor (I'm calling it Guts for mine) as an improv morale too, possibly, for some sort of non-damage dealing skill, like an intimidate or persuasion, the tougher guys are less likely to give up or be convinced as easily, etc. as they're theoretically better trained or experienced. I've got a few different variations, from more complicated/bookkeeping than FS, to less.
So your typical modern generic thugs, or basic unnamed bad/good guys, might be:
Name: 9 Modern Mooks (from book, slightly modified for compliance)
Weapons: revolver 9 (6 cap), knife 7
Attr: Bod 5, Chi 0, Mnd 4, Ref 5
NC Skills: Intimidation 8, Info/Crime 6
Combat Skills: Martial Arts 6, Guns 8
Combat Stats: KO 5, Guts 2, Edge 0, Life 45
Extras: none
Attacks (starting): 9
Threat: (8+6=14) + 3 + 4 + 26 + 7 + 23 + 0 = 77
Citizen Equivalent: 1.75
Gear:
And further back:
Name: 9 Ancient Henchmen (from book, slightly modified for compliance)
Weapons: Spear 10, Bow 7
Attributes: Bod 6, Chi 0, Mnd 4, Ref 6
NC Skills: Intimidation 8, Info/Crime 5
Combat Skills: Martial Arts 8
Combat Stats: KO 5, Guts 2, Edge 0, Life 45
Extras: none
Attacks (starting): 9
Threat: 8 + 4 + 4 + 16 + 7 + 23 + 0 = 62
Citizen Equivalent: 1.41
Gear:
And from the FS core book, Poison Thorns and Fast Eddie's Boys:
Name: 9 Poison Thorns (modified for compliance)
Weapons: revolver 9 (6 cap), machete 9
NonCbt Skills: Intimidate 6, Info/Crime 6
Attr: Bod 6, Chi 0, Mnd 4, Ref 6
Combat Skills: Guns 7, MA 7
Combat Stats: KO 5, Guts 2, Edge 0, Life 45
Extras: none = 0
Attacks (starting): 9
Threat: (9+6=15) + 4 + 4 + 28 + 14 + 23 + 0 = 88
Citizen Equivalent: 2.00
Gear:
Name: 9 Fast Eddie's Boys (modified for compliance)
Weapons: Morinco 6 (7+1 cap), Mini Uzi 6 (25 cap), machete 5
NonCbt Skills: Intimidate 6, Info/Crime 6
Attr: Bod 7, Chi 0, Mnd 4, Ref 7
Combat Skills: Guns 8, MA 8
Combat Stats: KO 5, Guts 2, Edge 0, Life 45
Extras: none = 0
Attacks (starting): 9
Threat: (5+16=21) + 4 + 4 + 32 + 7 + 23 + 0 = 91
Citizen Equivalent: 2.07
Gear:
And for the heck of it, essentially the least competent mook group usually encountered:
Name: 9 Normal Citizens
Weapons: revolver 7 (cap 6), baseball bat 5
NonCbt Skills: Info/Profession 6, Info/Everything Else 5
Attr: Bod 3, Chi 0, Mnd 3, Ref 3
Combat Skills: Guns or Martial Arts 3
Combat Stats: KO 4, Guts 2, Edge 0, Life 36
Extras: none = 0
Attacks (starting): 9
Threat: (6+6=12) + 3 + 2 + 3 + 6 + 18 + 0 = 44
Citizen Equivalent: 9
Gear:
DESCRIPTIONS:
KO = Outcome needed for one mook in the group to be taken out (5+ for basic mooks)
Guts = Outcome minus KO, divided by this, equals extra mooks KO'd (2 in Queex's example)
Edge = Life divided by Guts plus this equals the number of attacks the group gets on their turn (basic mook groups have 0 in this)
Life = the life, duration-wise, of the group itself, rather than life as a measure of physical vitality
Extras: any useful group abilities, schticks, etc.
Attacks (starting): how many times the GM acts or rolls to attack the PC's when it's his appropriate Shot)
Threat: Overall danger presented by this group as a whole - larger or more capable groups both increase this
Citizen Equivalent: basically how many normal people a mook is "equal to"
Gear: any non-combat stuff a mook group might have, either individually or as a whole
FORMULAS:
Weapon Value: (tot weapon damages) + (tot weapon capacities / number of guns) divided by number of weapons
Non-Cbt Skills Value: add together and divide by number of skills +1 for NC Skill Value
Attributes Value: add together and divide by 4 for Attributes Value
Combat Skills Value: add together for Combat Skills Value
Combat Stats Value: add KO + Guts + Edge and divide by 3 for Combat Stats Value, plus Life / 2
Extras Value: any useful group abilities, schticks, etc. add values together and divide by number of extras
Attacks: Life / KO
Threat = Weapon Value + NC Skills Value + Attributes Value + Combat Skills Value + Combat Stats Value + Extra Value
Citizen Equivalent = group's total Threat rating divided by 5