Just as a point of reference. Here are the major factors that affect the life of incandescent lamps in automotive applications:
1) Vibration and shock
2) Ambient temperature in the headlights or luminaries
3) Power supply
ad 1) By increasing the wattage you go for a filament type lamp that burns at a higher rate, i.e. becomes thinner more quickly and consequently becomes more vulnerable to shock and vibration
ad 2) Self-evident, particularly as filament lamps by their very nature are heat radiators that also happen to radiate at visible wavelengths
ad 3)By upgrading the harness to lower gauge(i.e. lower resistance ) wiring you protect it from overheating. This is just to save the wiring, not the bulbs. Too little resistance in the wiring is also detrimental, as bulbs are designed for and tested at 13.2V. The typical alternator voltage is 14.5V. Wiring resistance is thus included in the design.
I know it's all anorak stuff, but I've learned these things when my lights started giving me problems. I used to buy PIAA bulbs at £15 each. They lasted two-three months and would go without any apparent reason. Rather than splashing out on new headlight bulbs I installed new lamps - Lightforce - that use a completely different bulb (100W Osram Xenophot) with average lamp life of 2000 hours.
Hope it helps,