I'm not an expert on the topic but have been researching lighting additions for one of my cars and found that higher wattage bulbs (over the rated capacity for standard light fixture) suffer due to a couple of reasons: 1) the wiring harness does not deliver enough voltage to properly drive the lights because the wires are too small for the bulbs and not permit enough voltage/amperage to reach the bulbs. Running the bulbs under their power requirements will cause them to not light up as brightly as they are made to do. According to what I've read this reduces the lifetime of the bulb; 2) The excess heat buildup can also shorten the bulb's life and cause a meltdown in the lighting fixture and/or wiring harness and/or fuse box, and/or lighting switch. The most susceptible in this list to meltdown will go first!
The solution that was recommended to me, and what I'm planning on doing is to run a separate wiring harness rated for the wattage of the bulbs I want to use to the lights, and use a different switch for turning these on and off, plus using a relay so that the switch doesn't have to take the full brunt of the voltage. I was also told to have relay secton of the the light switch run through the ignition so that I don't inadvertently leave the lights on when I park the car (which I've done before with another car)!
Lastly, someone told me that the hella bulbs are longer lived than the PIAAs, FWIW.