The z4 is the best firmware version I.e running code and almost the best maps. Only superseded by the slightly better 6k from the sti 2. Which was the same map used in UK prodrive cars.
Once chipped using the spare slot on the motherboard (which was used to get around a group n rally regulation) the ecu fully takes on the guise of the chip.
However this is old technology and a crude/slow way of remapping (hence why many tuners will tell you it can't be done). It is effective but not as good as a car specific remap as the tolerance are wide to cater for minor vehicle variation.
Great value for money though if from a reputable source
Esl provide a daughter board that fits into the same location as the chip but the intelligence on this brings the ecu into the 21st century and allows laptop driven bespoke mapping but retaining all the standard ecu safety features. But at a cost of £350 + mapping fee's.