I haven't done the reprogramming yet. Mine is only 1400 miles (around 2240 KM), and I haven't taken my G37 back to dealer. I plan to do the upgrade, but I'll watch the car's transmission behavior for a while.
My G37 also has this issue. Most of the time I used manual mode. And when down-shifting, sometimes the car will just stop like hitting the wall. Quite a few people were complaining this issue in the myG37 site. Some of the them said that the issue will disappear after driving for a while because G37's firmware is "learning" how you drive the car. But for manual mode, it doesn't seem to "learn" very well.
Anyway, I'll give you an update if I do the upgrade and feel the difference.
I took my car to dealer today. The agent told me that my car has already had the new firmware! That's a surprise! It means the new firmware would NOT solve this problem!
Looks like we have no choice but to get used to it!