Different brands have tighter tolerances in the manufacturing process than others, a lot of factors influence a magnets strength and without an i-gauss meter (which I am still looking for) it's hard to tell precisely but given its a relatively cheap motor I am not surprised there is a large variance between a good and not so good motor. That said even a "not so good" Trackstar V2 I have seen is pushing better numbers than a few of the dearer motors I have seen.
As for shimming. Trial and error, remove the brass spacers on the rotors, install smaller shims and dyno each time to see the effect. There's no absolute answer, it's trial and error and tedious as all hell, you can spend hours using a $170 Eagle Racing Dyno, RC Crew Chief software and a laptop going through a bag of shims to get a result. Some motors it does little, some it can be up to a 3-4% gain. I do it because that sort of thing interests me, but will it win you a race? Maybe if you never crash and hit every apex perfectly every time.