Mine, according to a volt meter as well as a bluetooth OBD2 reader (smartphone interface), on a brand new 140 amp alternator...... starts out around 14.1 and drops back to around 13.3ish. If I'm running both rear and front fans, 13.1 is not uncommon. I actually don't think this is what it should be, and my previous alternator (same 140 amp crap from Autozone) would drop as low as 12.4 on an old battery, 12.8 on a new battery. Neither was good so I warrantied it out, and the new on is what I have now that will sometimes get down to 13.1.
If you actually are throwing 15.0 volts but not much higher, I wouldn't be terribly concerned about it. I've had other GM's (admittedly very different systems) that ran 14.8, 14.9 for years.