On the subject of fins: To get a bit of an idea of what I can probably do for the stock bumper - check out the Arkym M-Sport diffuser:
http://www.e90post.com/forums/showthread.php?t=471361
Last pic on the original post - if I come out from the bumper a bit around the edge of the diffuser that'll give enough room to come inwards and leave space for the fins...
What do you think?