On the O2 thing, I believe I already explained this to you, but it's not just as easy as the O2's pulling fuel via the trims, which I expect the enrichment would be outside the range of adjustment for at anything beyond very light load anyways, but I've got a few minutes, so I'll try and explain it again:
An engine's fuel consumption for power produced is called BSFC. Brake-Specific Fuel Consumption. This is not universal, some engines are more or less efficient than others but for the sake of this discussion, since both engines are SOHC Subaru boxer engines let's assume that the BSFC figures are the same for both engines.
On a MAF equipped car the ECM looks at the volume of air consumed and matches it with the fuel value it sees in the table, this is how it discerns load (air consumption). So displacement doesn't matter, it knows the air volume and can subsequently align the fuel accordingly.
On a MAP (Speed Density) equipped car load is determined from manifold vacuum, or rather, the lack thereof. This means the ECM fuels based on a table for a given alignment of vacuum, temperature and RPM.
So, at idle, both engines are going to have very close BSFC's. As you increase load, the distance between the program and the anticipated BSFC will widen. Why? Because let's say for the sake of argument your 2.5L made 160HP at WOT at 4,500RPM. The fuel table would be configured to properly satisfy the enrichment requirements of 160HP. Now, let's say we've swapped in an engine that makes 110HP at WOT at 4,500RPM. The engine is still being provided with fuel to feed 160HP because it only looks at load (vacuum), RPM and temperature to determine the fuelling requirements. Also, HEGO input is typically ignored under WOT and you you go out of base table.
So, your O2's will be switching and functioning under light load where the table is reasonably close and the trims will likely be able to compensate. But when you get on it, it will over fuel significantly because it's trying to satisfy the BSFC requirements of a 1/2L larger engine that produced more HP.
Does that make sense?
