There are different mass airflow ranges, so A is at idle, while C and D are learned more cruise-type driving (D is also applied in open loop at WOT but not learned there). For 08+ STi, trims should be within around +/- 8% - sometimes a touch worse (due to the factory FPR setup). All others should be within +/- 5% of so.
Boost leaks tend to show up as extreme negative values, especially in C and D (sometimes D is disabled by map and will be always zero). A bad MAF (esp. with older cars) tends to show extreme positive values. Front O2 can be erratic - hard to predict based on the type of trims. Vacuum leaks will show as extreme positive in lower ranges generally.