As OLDYELLER says, you'd need to know the exact construction of the light, but I'd be reluctant to chance it. Unlike most car headlamp systems, I doubt that anything in an Evo X is relay-protected. So running a 20w in place of 10w would probably mean you're doubling amperage through the entire system. This is not just a matter of more heat from the bulb. I think the Evo X uses a 6V battery, so with a 10W bulb, it's drawing 1.66 amps. Doubling that could easily burn wires, connectors, switches, and whatever else conducts electricity in there. And don't be standing in any puddles when you operate the switch.