I dont think ive seen anywhere about them running lean and cause the car to jerk purely because of a pod filter installed because of 'extra air'. Thats what the AFM is in there for What mechanic did you take it to? Do a search through here for reputable mechanics that are recommended to work on WRX's , and see what they say.

How well does it idle?

I was under the impression that a screwed AFM would mess the idle around, but through acceleration would be fine. It would just make your car run lean and boom!

In any case, if it were me i would try a new AFM (cheap compared to a motor rebuild!), and if that sorts it, all well and good. But I run a K&N pod filter (it was already installed when i bought it) and Ive had no trouble with it. From reading a lot of threads about it, the K&N filter should be fine, as long as its not over oiled. Perhaps even buy the oiling kit yourself, and give the filter a clean and lightly oil it yourself if youre not sure.