Yes, I do let it warm up for a few minutes..Silly question, but are you letting it warm up any prior to driving to/from work, or are you just hopping in, turning it on, and heading out? Reason I ask is because it takes the car a little bit to warm up the O2 sensor and start using it for AFR readings. For me, it usually takes a mile or so before I notice the AFR stabilizing while cruising in the morning.
Wonder if it's just the computer running it REALLY rich for most of your commute and it's killing your mileage average. E30 doesn't burn as hot as gasoline either, if I remember correctly, so it could take longer to heat up the O2 sensor.