I didn't knew that there was an interace only jar of the stax api. It seems small enough to add it as hard depenency then.
But I just compared XMLStreamReader with XMLPullParser from Android and found that they are not 100% equal. Which leaves us with two options: Use XMLStreamReader as interface for Smack and implement the missing Android methods as empty methods. Not really a good approach, because it's error prone having an interface where not all methods are usable.
Or create a new interface that implements the interesection of the methods between the two interfaces. That is the option I would go with.
Performance wise, I don't see your point. You would still use the Android parser. It's just accessed through another interface.
There is a huge difference if you use the bytecode of a class that is distirbuted with your Android application or if you use the API provided by dalvik. You have to take for example JIT, optimization and caching effects into account.