Newbie butting in here with my two pennies...
I don't understand why anyone would want to drop an E89?! Once upon a time I had a Corrado VR6 which I decided to lower by 40mm on coilovers because as standard it looked like it was on stilts and the appearance was vastly improved, but the stance on the Z4 is already perfection in my opinion. The suspension is highly sophisticated and the ride is already rather firm (especially on 19s) with very little body roll and it seems like madness to me to 'downgrade' to what would be essentially inferior springs. I have seen some cars that are so low they couldn't even clear a baby hedgehog and I just don't get it... but then I am middle-aged. Each to their own I suppose.