I have a piece of communications gear in my Camry with an accurate voltmeter as part of the control head display and it has always shown 14.2 with the engine running. Voltage quickly drops to 12.8 with the engine off. 15 volts is at the high end for a lead acid battery, though AGMs can run a little higher. Electric motors draw a bit less current at higher voltage which puts less stress on the windings. Your new ignition switch may have lower internal resistance than the old one, but you should check the ground path for the alternator which is usually the case, through the mounts to the engine block, and then through the car body back to the negative battery terminal. Sometimes a direct ground strap from the case to the body helps. The 8 valve SAAB 900s had an alternator bracket about a foot and a half long, a common practice was to add a more direct path to the body with a hunk of braid. An alternator case which is slightly "above ground potential" can produce excess voltage.
