I've had a DC Power HO alternator in my car for about 3 years now, and it charges at around 15-15.5v at all times. I haven't had any issues, but at the same time I run all dry cell/AGM batteries in my car (one in front and one in the trunk), so they may like higher voltages than standard batteries can take.

So IMO it's up for grabs whether or not charging above 15v is safe for your batteries.