Re-Reading the OP's post...... Reset the PFC!
You don't know what
MAF was used for the previous tune & many people fit Z32
MAF sensors to Subarus for better reliability & higher resolution.
The problem is that the Z32 airflow/output voltage curve is completely different to the stock
MAF.
Most tuners adjust the airflow curves (accessed by either a datalogit or Apexi software) to suit the Z32
MAF which will result in your motor being running extremely lean with a factory
MAF sensor.
To be safe, I'd reset the power FC to the base map first (settings -> -> initialize)
Make sure you turn 'on' the boost control under ETC -> function select -> 1. Boost cntl kit - the characters are Japanese
This will help
Go back to the Settings tab navigate to Settings ->Boost
You will end up here:
Set boost #1 to .6 & 15 - press the left arrow to exit this menu
(this step will allow the PFC to boost cut if the boost spikes)
Also be sure to perform the proper 30 minute idle learn which is:
(you must not touch the throttle at all during this process)
1. 10 mins idling from a cold start with no electrical load -lights, a/c & rear demister all off
2. 10 mins idling with head lamps & rear demister switched on
3. 10 mins with head lamps, rear demist, A/c on & the fan running flat out
Failure to do this will result in a car that stalls or has an erratic idle when you pull up to traffic lights etc.
Have fun but don't lean on it until it's been fully tuned!