What stats do you use? Is it straight points or are powerplay points used? If you use PP then Gonchar is far more valuable then Timmonen. Gonchar is the #1 PP QB in Pittsburgh where Timmonen is not now that Pronger is there.
Pominville is not that much lower then St. Louis. If St. Louis can return to 100 point form then yes he is move valuable then Pommer but based on the last two seasons it would seem that St. Louis is more of a 80-85 point player. Pominville had an off year and should return to 75-80 point form.
So if your deal involves PP points I wouldn't do the deal. Gonchar out scores Timonen by 15-20 points and runs the Pens power play which would make up for the difference St. Louis over Pommer would make.