yes it will void your warranty as you're modifying the power delivery circuit.
if your board dies for some other reason unrelated to the mod you may be able to clean the area well enough for it not to be detected. (although carefull inspection will probably detect carbon dust under the edge of the resistor or pencil marks in the solder...conductive ink can be removed with acetone)
the basic law of modding is don't do it unless you can afford to lose it.

you should be able to reduce the vdroop with just a pencil