Our bone stock and pampered 335 had similar charge pipe/intercooler oil buildup at 30k miles. Not uncommon for any N54 car that is driven, mostly it is attributed to oil vapors that make it out the valve cover ventilation port and into the rear turbos inlet. Alternative to this is that turbos can and do seep minor amounts of oil, even new, it is not atypical but it should be a very small amount over actual mileage- meaning your consumption should be very reasonable if everything is healthy.
The stumbling is probably your injectors flaking out, could be the cause of the vapor like smoke as well. Crazy the price bumped to $1,200 for the set, they were about $800 a month ago...
https://www.fcpeuro.com/products/bmw...SABEgJ13vD_BwE
All in all you really want to mostly concern yourself with oil consumption rates, if you are losing serious amounts of oil in low amounts of miles then you should be more quick to raise the eye brows (ie. 1 qt or worse per 1,000 miles). Even if you are losing oil you also want to make sure it is not some external leakage (ie. oil pan, valve cover, oil cooler thermostat housing, etc etc), as obviously that can be a large factor as well in "consumption" rates. Also make sure you are changing your oil at reasonable intervals (ie. every 3-5k miles), especially if you run the car hard.
PS: The RB OEM Turbos are not really "upgraded" per se; but they do have better internal bearings.
Thanks,
Rob