i wouldn't worry about the value of study-abroad for law school. worry about the value of study-abroad for life. studying abroad was without a doubt one of the best experiences i have ever had, and it really changed my perspective (and my life plans). i think you should do it, even assuming your gpa takes a hit (which you're assuming it will--who knows? maybe you'll be part of the 19%). study like mad for the lsat, score in the 170s, and realize that it's more important in the long run to be a well-developed person than to go to harvard law.
if you're really more concerned about your prospects, if you're at all interested in working abroad, firms tend to prefer people with experience abroad for that (at least, the firm i'm working for right now does).