I dont have any Subarus in the family anymore so I'm not so I'm no familiar with this, but unless this is a popular engine swap that many people on some forum can attest to working ok, I don't see this ending well. Smaller engine with significantly less power, running off the same gearbox ratios....and I have serious doubts the ECU runs the same tuning maps between the 2.0 and 2.5 liter version.
I was into the engine swap scene back in the 90's and early 2000's for Toyotas and Nissans and Mazdas and sometimes you could swap out a 1.6 for a 1.8, or a 1.8 for a 2.5 or whatever, but there were always some small, but significant details that had to be worked to make it all jive correctly, even if it all bolted up ok. You had to use the appropriate ECU, or manipulate the airflow meter readings with a piggyback, or.... something. It was never plug and play. Even if the wiring harness was the same, using the original ECU or airflow meter between different displacements of the same engine family made the the air/fuel ratios way off, cats were glowing and melting, the transmissions wouldn't shift right, idle would be all over the place, check engine lights galore, etc. It can be done and corrected, but you have to know which sensors carried over and which didn't, which ECU versions would run it ok and which didn't, etc etc.
I want this to work out, but its not sounding like all the homework has been done. Let us know how it goes, hopefully this turns out ok.