I'm about to pull the trigger on an underdrive pulley for my 550 and was wondering if anybody who has performed this mod has positive or negative comments regarding the pulley? I know this is not a super popular modification for non-M cars, so I am specifically interested in hearing from the non-M car owners out there who have done this mod.

Here is one for the 535/545, but I think I'll give it a shot on the 550 unless anybody has evidence why it wouldn't work on the 550.

Here is another photo of it:

 

i was also looking aat that for my 550i

 

I had them on my e46 they were ok not a really huge noticable difference. My only tip for you is to make sure you don't get a kit with a pulley for the alternator, our cars have far too much electronic stuff to afford any less power from the alternator.