With the stock MY 98 2.5 there was almost NO lag. You could hear the turbo wind up at 1800 RPM, and could feel it at 2200 RPM. No I start to "feel" it around 2500 RPM,but I also had to up the boost to get the same Speed over distance numbers.
I think the SOHC heads will be a LOT easier to fiend than the DOHC heads though.
Yes, the bore on the 2.2 cyl is smaller that the 2.5. But if you use the 2.5 steel gasket, it is so small that it does not affect the "squish" area. If the gasket was more than 1.5 mm thick, it would have left a big enough gap to interfere, but it's not.
I would say that if you already have SOHC heads, just put them on the EJ22T short block BUT keep in mind that your compretion will be WAY lower than if you go with the DOHC heads. Again, it depends what you want as a end result. If you are gonna high rev and high boost (20 + lbs) go with the SOHC heads or get a Honda
otherwise go with the DOHC heads.