Suber - the reflector design could definitely be the problem. I've noticed that the little "hat" that sits on the top of the bulb ("top" being the point, which really aims straight ahead, not up, if you know what I mean) has gotten discolored. Looks like it's being overheated by the higher wattage bulbs - that says to me that a lot of the energy of the bulb is being wasted.
'99 Legacy 2.5GT (WNTR-GT): removed intake silencer, K&N filter, 18mm SPD rear bar, 80/100W bulbs with heavy duty wiring, 205/55ZR-16 Yoko Nexus tires
Supercharged '89 Mustang GT convertible (SUMR-GT): 9# intercooled ATI supercharger, etc, etc...