sorry for the late reply. Unfortuately the 128i is my wife's and I don't drive it much at all. I haven't notice a late/slipping engagement into third. Does it only happen when manually shfted? The manual shift should not be any different than the automatically trigger shift. Seems like you have a problem. Best to drive another one and see if it's the same and if it's different you can show the service rep the difference. I'd get that clarified before the warrenty expires. Auto boxes are expensive to fix...
I'd posted the following question a couple of days after you posted to the "Q about Steptronic with shift paddles" thread but hadn't seen answer. Figuring you'd probably not visited the thread after your post, thought send the question along as PM.
"I have the Steptronic in my '13 128i as well and find it a very good automatic with only one annoying trait. When manually shifting from 2nd to 3rd between about 2000+ RPM and 4000+ RPM the shift is noticeably slow and there is a pronounced engine speed overshoot before 3rd engages. It doesn't do this in a 2nd to 3rd auto shift or in any other gear change, auto or manual. Have you experienced this in your Steptronic 128? If so, have you reported it to your dealer and what happened? I did report it at the first service and was told it was 'normal', though I have my doubts since I do not see how the manually triggered 2-3 shift would be any different than an automatically triggered 2-3 shift."