The V29.2 Progman update was designed to minimise, or at best eradicate, the rattling wastegate issue that was being experienced by quite a few 335i drivers. Essentially, what was happening was that at low revs the engine management closes the wastegates - however some wastegates suffered from a deterioration in freedom of movement, and therefore didn't close 100%. Because they weren't sealing properly, they were rattling away when the throttle moved from an open to fully closed position.
The solution BMW came up with was to reprogram the ECU so that it would keep the wastegates
open until 3000 revs, therefore reducing the rattling. However, the downside to this is that the engine suffers from increased turbo lag at low revs, and hence the dispute in the USA started with drivers complaining that the V29.2
reduced the drivability and performance of the 335i.
In practice, there is a noticeable increase in turbo lag with the V29.2 software (i had it on my car a few months ago, then changed back to V28.2 software because of this). BMW however realised that the software change did not fix the inherent wear problem with the wastegates, and so have recently issued an internal PUMA report to instruct dealers to replace the wastegates if there is sign of considerable rattling from the engine.
The good news is that BMW are now on V31.1 Progman software, and I can report that the drivability is the same as that when I had V28.x.
Bottom line - your car is going in for service and a software update - the software will be updated to V31.1,
and you should have no problems or worries 