I understand that the stock ECU has a "learning" capability, but to what extend (in terms of other performance modifications) it needs to be reprogramed or replaced with an aftermarket ECU?

 

I think you should state what aftermarket modifications you are planning so the gurus can gice you better advice.

 

CC6:

As Steve said, ECU's are reprogrammed with the desired HP in mind and the hardware required for that power level.

One HP level retains the stock K-16 turbos and adds a front oil cooler, but further levels are mapped around and require; different turbos, intercoolers, fuel pressure regulators and things like that.

Tell the people who provide the ECU services what power you desire and what octane fuel you have, and they will tell you what additional hardware you need.