I've been running a mix of BP Ultimate and Shell V-Power for a year (depending on which servo is closed when I'm at 1/4 tank).
Fuel economy is generally under 10 l/100k city (6MT 320i).
Switched to a tank of Shell 95 to see whether my fuel economy changes. Notice more sluggishness in first and second gear. After 1/4 tank gone usage is 11 l/100k.
Personal experience gives BP Ultimate the best economy and smoothest engine revs.
Once I do the sums I'll probably revert to BP Ultimate exclusively. Wish BP would discount more.