e91 here. from my experience, the reason the front doesn't go as low as the rear is because when you air out the sway bar + end link holds the car up.
Basically, if you draw a line from the connection point of the end link to the strut all the way down the end link, then from the end link connection to the sway bar to the bend in the sway bar, this is all the length you have to work with. if you increase the length of the end link you can go lower.
here is a crude diagram i just made:
I made longer end links (by making a longer center bar) and now my car slams nicely. I sold a pair of the longer center bars for the end links to another member (username PRO.) on the forum and maybe they want to chime in.
Here are some pictures:
Before lengthening end links, this is a low as the front would get:
I removed the end links, and this is how low it would get:
This is the car now with legthened end links and as low as it gets:
here is one of my older posts:
We lengthened the end links to their max (10mm of threads inside each end link head). Length was set to ~285mm and the car sits much lower, but about 20-25mm higher than without end links installed. This is satisfactory for now, but will want it to go lower.
I need a longer center bar for the end links so that I can set the length to 310mm or so. Current center bar is about 162mm long.
It was counter-intuitive to lengthen the end links when lowering the car, but this has worked.
Update:
I fully understand how this works now.
The top end link connection point on the strut is a static point, no matter the height of the car. When the air bags deflate and the car lowers, the sway bar (and bushings) lowers too; this causes the end link connection to the sway bar to pull on the sway bar and it pivots. If your end link is too short, when you air out your end link is actually holding the car up; the weight of the front of the car is suspended on the end link and sway bar. The longer the end link, the lower you can go (to a certain point).
I think an end link length of 310mm-320mm is ideal. This will allow you to go as low as the car wants to without end links attached. I have not checked clearance for this length yet though as the end links that came with the kit can only go to ~285mm (while still leaving enough threads, ~13mm, inside the end link heads so the connection is strong and effective). I have clearance with this setting. --- Keep in mind I have an e91, so your results may vary.