You guys might find this
webpage from Illinois Capacitors interesting. It has capacitor Life Calculators for electrolytic, film and ceramic capacitors.
It quantifies the advice we've seen nicely. Voltage derating does indeed increase the life span. Using 105C rate caps extends them considerably more though.
Typical general purpose caps have a 1,000-2,000 hour life at maximum temp and voltage.
Nichicon has a new line out of high ripple, long life caps rated for 450 volts, 105C and 10,000 hours. They also have a very small diameter and great for restuffing cans.
Use one of those at 300 volts, keep the temp down to 50C (122 F) and it should last about 675,000 hours or 77 years of continuous operation
Using a cap with the same ratings except 85C should last about 169,706 hours or 19 years of continuous operation.