I ran 89/90 (US ratings) octane in my 535i for a couple years and for the past few months I've used 87 octane and I am getting the same MPG either way. From my measurements, the difference is from 0% to 3% better and the cost of the fuel is more than 3% higher.
Also, there is no pinging problem and its not chipped.
For 8 month period May to December:
Year--Octane--Avg.MPG--Miles Driven
2003-- 89/90 ---20.17 ---- 13394
2004-- 87 ------20.09 ---- 8237
Therefore, it is not worth the extra expense to use higher octance than the minimum required for US spec cars without perf chips.
DanH
1989 535iA 254k Miles
1992 535iM 330k Miles