I can see it hurting performance, actually. From what I've read, the H6 engines use a variable flow muffler which decreases backpressure by use of a valve in the muffler. If it's mechanically actuated, then it opens by means of exhaust pressure. A 2.5L engine (or 2.2L or whatever) will put out less exhaust gas for a given rpm which might destroy any gains made by the system on the 3.0L engine (I'm not sure of this).
If the exhaust is activated by the ecu, you'll DEFINITELY have problems