If your bulb exceeds the OE wattage, you could be in for problems related to your wiring harness, and I wouldn't do it.
BTW, a 100W bulb cannot "really" be 80W. Wattage is a physical measure, and is not a measure of output, which is actually measured in lumens.
Now, look at compact flouro-lights. They are rated for a much lower wattage than standard light bulbs, but are rated for the same lumens at a much lower consumption.
1998 C230 "Black Betty"