lux - I've noticed also that there is a lag before the step kicks down. One theory that I have is that I am typically a pretty passive driver and I don't lay into it much. Because the software "learns" our driving style and applies the shift patterns accordingly, I just assumed that may have something to do with it. However, I think that algorithm is based on the last 50 or so shifts - so if you've been driving aggressively for a while it should be past that.
Have you tried the same driving style with the step in sports mode?
The one thing that still bugs me is the counter-intuative manual shifting for steptronic. Pushing forward to downshift and pulling rearward to upshift the stupidest thing I've ever heard of. I used to use the manual mode from time to time in my other cars, but I can't stand it in the E90 and I can't get used to it. I'd love to know who's the dumb Kraut that came up with that brainstorm. No wonder they can't win a war!
EDIT: lux, are you doing this in manual mode? No wonder! In M the shift will only occur to prevent engine/transmission damage or when they become grossly out of sync. I would recommend DS mode for this type of driving.