if you try to run the 2.5 on a stock 2.0 ECU, it will be sluggish and struggle in area it normally wouldn't, an you may be faced with engine problem if driven like that over any extended period of time,such as premature wear on the cams,valves,bearings and timing set. The stock 2.0 ECU won't allow the 2.5 sufficient air and fuel which can stress those parts over time. You may be able to buy a custom chip to make up the difference.