My car gets 23-25 but, I don't do much in town driving. I would definately do the O2 sensor. Buy the $12 tool (slotted socket) and it is an easy job. I also bought the generic sensor with connectors. It is a cut and splice but pretty much a no brainer.
If you are roddin it round town though, don't expect better than about 15.
Current ride... '13 GTI Wolfsburg
BMW's from my past...
'92 535i/5 '89 325ix/5
'98 750iL '86 325(e)/5
'95 740iL '84 318i/5