20 sounds like a good gain, if that's possible. The largest gain I have seen on a N/a E90 is around 15. Everyone else has said that anymore is reaching.
Keep us updated if you get anymore news.
M-tech e90 - RIP
11.5 sec 1/4 mile @ 122mph N54 135i - RIP
e53 X5 4.8is - The beast!