i can't think that a real rain jacket would likely be needed in August in the High Sierra. most likely it's afternoon thunderstorms, and even if it rained for two days, a light poncho would still be fine (if you have a real shelter, especially). it's unlikely to be cold enough to wear much more than shorts and a poncho and a coupla insulations layers.
gotta love the Sierra in summertime. you need to be prepared, but (in general) it's also a great time to go pretty dang light.
"head-to-toe goretex for the JMT"
i dunno about that. goretex doesn't work very well, and i'd never think it would be cold enough to worry about rain pants (but my legs never really get cold if my torso is fine). i'd be thinking about a very light poncho, or a light rain jacket (but then you need a pack liner/cover probably), for "usual" summertime weather on the JMT. worse case scenario, you hike on, warm and wet-ish, under your poncho or jacket, and set up a shelter early if necessary. i know there's always a chance for "weather" but man the summertime Sierra is almost always seriously mild in the big scheme of things.
have a great hike! i hope to be somewhere south of there off trail on the granite for a long hike in September!