even with a new alternator, the voltage output is above 15V?

.5 volage over is still acceptable. Were you having an issue and is this why you decided to look at the alternator? I did find this though.

Installation note:
The positive power supply of the preheating control unit is also connected to terminal 30 of the alternator. These lines must be connected without fail, otherwise the preheating system will fail.

Also, if you allow the engine to run for about 15 minutes does the voltage go down? And if so, what does it go down to?

My battery was drained so I replaced it. A week later the battery drained again, and I looked into the voltage from the alternator. it was at 15.3V (the original BMW alternator). I changed the voltage regulator (the brushes were finished). then the voltage dropped to 15V. I decided to change the alternator all together and put a new one in, thinking it is probably old. The new alternator still outputs 15V (220Amp alternator). Everywhere I look they all say 15V is not very good for the electrical system. After 20 minutes of runtime, the voltage is still at 15V. The power supply of the preheat unit is also connected to the alternator. There are two cables that connect to the positive terminal of the alternator.

modern BMWs should generally be between 13.7V and 14.8V, so if it as at 15 volts it may cause damage to modules, but it can be charging this way due to an electrical draw. Also, if you allow it to sit and run for about 15 minutes what is the voltage?