Been studying a few more images and the difference appears much more subtle.
Here is a high resolution of the 540i
http://images.car.bauercdn.com/pagef...438/zm-002.jpg
This youtube video has a nice close up of the 550i rear end (see 08:84 minutes in)
The only difference I can spot is that exhaust tip cutouts on the 540i look more angular and the ones on the 550i appear more flat on the top.
The 550i diffuser part 51128067763 looks to be a ~£100 part and does look to fit the 540i bumper. I think I'm going to go for it!