100W of LED is way more than is necessary for vehicle headlights.
In regards to luminous efficacy, lumens/watt, essentially, lamps are as follows:
Halogen: ~15-20 lm/w
Xenon Arc Tube: 15-60 lm/w
LED: 80-200lm/w
Stock Halogens, or even aftermarket, @ 55W per bulb will output up to 1100 lm. 2200lm at best, for both bulbs.
Xenon arc tube, usually 35W per bulb, output somewhere around 1600-2000 lm. 3200+ lms for both.
LEDs, in order to achieve the same performance as those Xenon arc tubes, would have to be 20W/bulb.
Heat is not really an issue per se, but you do need to keep LEDs below 85°C junction temperature for their 50,000hr L70 lifetime to be maintained. This isn't particularly difficult when you only have ~15w of heat to remove via convection. It does however become an issue when you decide to run 50W of LEDs in a single headlamp fixture. And, it's not the fixture that will have any problems. The fixture can handle 55W of heat being created inside it, because that is what those 55W halogens are. It's a problem for the LEDs themselves and removing heat from the chips via heat sinks.
So, you don't have to worry about the headlight housing being damaged, you have to worry about the LED bulb replacement being damaged.
So, yeah, stick with a 15-25W replacement and you will be just fine.
For reference, some LED fixtures I'm working on at work right now are 1,800W fixtures. (3) 600W power supplies, over 300 LED chips, and the hea tsink is cooled by fans providing about 250 CFM of airflow across the entire fixture. The heat sink is 3" wide x 144" long x 2.5" tall.