I think it is well worth the upgrade - there is still a bit of turbo lag that I sense when I floor it from a stand still but huge boost edge when your cruising on the freeway and then floor it in the sport+ mode. It was a great deal in my view - close to having an AWD M5 - just missing the benefit of a DCT. I will tell you though the springs are the best thing when doing a simple comparison of individual modifications from the stock 550i - flattens the car out on turns and has a more aggressive stance with lowering. I also did the free flow exhaust - I'm not all that enamored with the Dinan tips though - might change those down the line.
Current - 2013 550i xdrive M sport carbon black Dinan S2, Dinan suspension, Dinan free flow exhaust
Retired - 2008 M3 space gray
Retired - 2000 M5 carbon black