well, the Garrote is (Str/2)p
he has a dice pool of 13 (Agility + skill)
so, IF he manages to surprise the target he can use the garrote, and it will do 3p+ net hits on the first attack, resisted by body + impact armor...
After that, you follow the subduing combat rules on page 161 of SR4a...
Basically, it becomes a game of complex actions, with the defender trying to get more successes then the hitman to get free (not likely because the initial attack was unresisted, and those are the initial target number for the defender to beat. The hitman can also spend a complex action to tighten his grip! however, a STRICK interpretation of the rules means that each complex action just inflicts base weapon damage (in this case 3) as the hits for the subdual test merely increases his hold to prevent escape.
So, in the first pass of combat, he is going to do on average 13/4 = 3 hits + STR/2p damage... or 6 points. resisted by Impact armor and body.
the next complex action he is going to do 3 more damage automatically, unless the defender has both a higher initiative AND breaks the hold.
*****
is it an overpowered weapon??? I guess that depends on his infiltration skills, the situation and a host of other factors.... but it seems inline with other things a player can/could do from an ambush position...