blows my mind how BMW can make the cars they do and have springs that break... But... here's what I discovered
Aftermarket springs listed for the E46 XI along with other models, as in you'll see "325i 325ci 325xi 330ci 330xi, etc" don't work with the xi (that's what Pelican told me).
There are actually 2 types of non appropriate "XI" springs (regular and heavy duty). Good luck getting the actual spring rates out of these dingleheimer manufactuers to do a real comparison. I'm sure a search here will find the standard rear spring rate... but also no guarantee.
All this considered, and the fact that a single rear spring was $175... I went eibach... Better to have a slighly lower slightly better handling car with quality springs than pay more for overpriced BMW or unknown aftermarket IMO...