This is the stock coil, correct?
If you know someone with an old fashioned osciliscope, you can hook it up and measure the "dwell". Just get a set of scrap ignition wires to jumper from the coil to the plug, you do not need the #1 reference if all you are doing is watching a single firing line. I have done this for diagnosis purposes in the past with good results on DIS cars. make sure you run a smal ground wire from the coil to the head, its probably not needed on a BMW, but its best not to take risks with electronics. On the older Jags, it would blow the instrument cluster if you cranked the car with a coil ungrounded.
Or even better, use a lab scope to check it, but not even many pros have them.
I would be surprised if running it at anything other than stock dwell would make much diference. BMW has always been a bit tight mouthed about this type of figure, so I am not sure its published anywhere.