LSD is the first and most important mod in my book. Otherwise, you will suffer from traction issues as you increase your power.
This is in my sig at the bottom:
http://www.e90post.com/forums/showthread.php?t=311570
You will wear out your rear brakes with your open diff and the nanny controls.
Coilovers won't help a car with traction issues; LSD will. Both will be fantastic!
IMO, get the diff, first.