Is 50 feet of 12-gauge extension cord overkill for a 15-amp table saw?

No, it’s fine. That cord is rated for 20 amps, so a 15-amp saw won’t stress it. Voltage drop is negligible—only 2-3 volts—and won’t affect performance. If you’re tripping breakers, it’s the saw’s startup surge or a weak breaker, not the cord. 12-gauge is standard for 15-amp tools at that distance.