As far as I know, when you call a shot, the target suffers the current effect from your level, as well as other effects from previous levels e.g. at 16th level, when you call shot, the target will lost -1 THAC0, -1 to save vs spells, -1 to strength and suffer an additional 2 points of damage. These *are* accumulative, therefore an improved haste archer attacking 9 times per round with called shot would easily take -9 away from the targets THAC0, -9 save vs spells, -9 strength and suffer an additional 18HP damage, therefore making the enemy extremely vunerable to spell effects and drastically reducing their to hit and damage rolls. Great for dragons
