There's a bit of inconsistency in the stages depending on whether you are talking MMC, G56 or Krazy Horse. Basically the standard ECU should be OK until you upgrade to the 569 cams.

MMC/G56:

Stage 1: Exhaust and air filter, fine with standard ECU
Stage 2: Add upgraded cams and ECU

KH:

Stage 1: Exhaust and air filter, retain standard ECU
Stage 2: Add ECU and dyno tune
Stage 3: Add cams and dyno tune
Stage 4: Add 128ci big bore kit, roller rockers, ported cylinder head, throttle bodies, high flow injectors and dyno tune

Red Leader drive