Those will fit nicely and you wont notice any difference at ride quality or any other problems. I have pre-lci e90 with 17x8 et10 at rear. The 18x8 et20 on lci would be the same as 18x8 et12 on pre-lci. Basically it will extend to same place. Here are some pictures of my winter setup 17x8 et20 front and et10 rear.
http://www.e90post.com/forums/showpo...9&postcount=30