I have no official source for the differences between the early APX and later BAM/BFV variants, but the biggest difference is EGR (Exhaust Gas Recirculation) on the APX and no variable camshaft adjustment.
You´re right.
APX -> BAM = 225PS
APX additionally has only one O2-Sensor precat, BAM/BFV equipped with two O2-Sensors (precat and after cat). APX fitted with narrow band O2 sensor precat, BAM/BFV fitted with wideband O2 sensor precat and narrow band after cat.
Additionally the turbo charger is a little bit different. APX uses originally the K04-020 or -022 and the BAM uses K04-023. The T/C is basically the same but the -023 is equipped with a EGT-probe which is also used in the ECU for fuel enrichment to cool down the engine at full load.
BVF is the 240PS variant (known as "Quattro Sport" - produced in limited numbers), only difference afaik is the raised boost pressure compared to the 225PS and some other things, non ecu related.
The ecu file of the APX is of course different, because there are a few more variables the BAM ECU has to handle.
The APX will be a little bit easier to modify if the hardware of the car was intensivly upgraded (e.g. bigger T/C , intercooler, bigger injectors, bigger MAF, stroker kit..) ;-)
