OK - to set the stage, I am talking about driving in 'S' mode and not automatic or 'D' mode.
I had an '09 E92 ///M3 and had a very noticeable lag when decelerating into a turn even when I had given the command to downshift. And if it was a left turn into traffic, it was very dangerous.
I now have an '11 E92 ///M3 - identical to my '09 except for ZCP - and there is no noticeable lag at all under the same conditions. There is a barely perceptible noise when the car is automatically downshifting (from S4 to S2) as I make a turn (yes, it does automatically downshift even in manual or 'S' mode), but this does not constitute a 'lag'. In fact it takes less time than would a manual (of which I own in other cars).
Don't know if this helps any. Good luck!