Low-beam DRLs operate at 60% voltage; high-beam DRLs at 50%.
Not quite right. Here are the requirements for low-beam DRLs as contained in Canada Motor Vehicle Safety Standard 108:
A daytime running lamp that is optically combined with a headlamp shall
(a) if combined with the lower beam of a headlamp that is designed to conform to the photometric requirements of this section, operate at
(i) normal operating voltage,
(ii) in the case of a DC (direct current) system, not less than 75% and not more than 92% of the normal operating voltage, or
(iii) in the case of an AC (alternating current) system or a modulated voltage system, the equivalent root mean square of not less than 75% and not more than 92% of the normal operating voltage;
(b) if combined with the lower beam of a headlamp that is designed to conform to the photometric requirements of section 108.1, operate at
(i) normal operating voltage,
(ii) in the case of a DC (direct current) system, not less than 86% and not more than 92% of the normal operating voltage, or
(iii) in the case of an AC (alternating current) system or a modulated voltage system, the equivalent root mean square of not less than 86% and not more than 92% of the normal operating voltage(…)
These are the standards manufacturers comply with for headlamp DRLs in Canada- and US-market vehicles. The "headlamps designed to conform to the photometric requirements of section 108.1" are ECE headlamps.
Bulb life varies exponentially with changes in voltage, to the -13th power
Yep. And bulb output (lumens) varies exponentially with change in voltage, to the +3.4 power. It's tempting to say the life curve is so much steeper than the output curve that it's worth lowering the voltage to get long lifespan, but it's really not. If you look at the data for how much more likely you are to be in a visibility-related crash after dark than in daylight, you will quickly decide that output is much more important than lifespan.
Silver Stars, like all other "extra white" bulbs with blue glass, are a scam; they produce
less light than a standard bulb (despite the "brighter! whiter!" hype) and they have an unreasonably short lifespan. Long-life bulbs greatly reduce the performance of the headlamps. First thing to do is stop buying bulbs from the dealer; it's a needless expense and they aren't any different from the name-brand bulbs you buy at the parts store.
If your low beams have a full-cap type of bulb shield (that completely blocks off the front of the bulb from view through the lens), then the Philips HIR2 is the bulb to pick for the low beams. It has much greater output than any 9006, at the same wattage, and an extremely long lifespan. The Philips Xtreme Power is the best 9006 on the market, but its lifespan is short.