Can't say I've noticed anything like you mention. Although I've only had my E90 for a couple months. I've just recently dialed it in with the correct JB4 map and E85 ratio. I usually flog it through a few gears at least once a day and it seems to be fairly consistent... FAST!
Current: 2015 M235i F22 BSM (FBO)
Current: 2015 X5 M Sport CBM (hers)
Previous: 2008 335i E90 Tiag (FBO)