The cause of subframe damage is poor welds that don't hold. Xi's dont have the bad welds, so no subframe issues.
Poor welds are poor welds. It doesn't matter how much/little power the rear wheels get- if the welds are bad, the welds are bad. You either have a cracked subframe, likely to get one in the future, or will most likely never get a cracked subframe.
Luckily all xi's fall into the third category, as do most e46's. It's the welder that determines if your car has potential subframe damage. E46's without poor welds won't have subframe issues. E46's with poor welds either have a cracked subframe, or the possibility of one in the future. Adding more power to a car with good welds isn't going to cause subframe damage. On the other hand, adding more power to a car with bad welds most likely increases the chances your subframe will crack.