Most remappers should be able to get 280+ bhp out of the 330d.
I would speak to .....
http://www.e90post.com/forums/member.php?u=32040
He has done loads of 330d's with good results and is based in the Midlands so not too far.
Given the choice I would always prefer a remap over a box, they actually tend to leave less of a trace than a box, and that is with the map still on there. Plus you can tune a lot more parameters too.