Gumball - I may be wrong, but look at it this way (and I'm just making up the numbers here): Let's say the STi struts are designed to damp the vibration of a 275#/in spring on a 2800# car. Now you put those same struts on a 3200# car with, say, 325#/in springs - that's going to put a significantly different load on the strut. So I think the struts are also affected by weight.
I think the reason the Impreza AGX struts will work on the Legacy is, as pointed out above, they are adjustable.
'99 Legacy 2.5GT 5speed sedan