Yeah, yellow springs. Can't find the thread now that discusses them. Pretty sure it's another Mr. 5 thread...
Edit - found it:
https://www.e90post.com/forums/showthread.php?t=237309
Here's one that covers the spring rates:
https://www.e90post.com/forums/showthread.php?t=235797
I think the drop is about 0.6" from m-sport. Though bear in mind that if you refresh the top mounts etc, it'll be less than 0.6" lower than you are now. You can raise it a bit with spring pads or camber fix plates, etc.
They're definitely a lot stiffer than eibach pro kit; ~148 Vs ~195lb/in. Stock is ~135. Remember bump stops are super important too, and way more difficult to quantify.