I feel that you should keep the rifle at 13lbs or more. I am in the school of thought that you should get stronger and build endurance to meet with whatever physical demands you may face. I am not saying that to downgrade you (could have medical problems, short suspense on a hunt, numerous other reasons). But I think hunters would enjoy a hunt more if they had a physical goal and worked toward it so they would not be sucking on the hunt. Not to mention you would feel better, live longer, and have a better standard of living. Plus with the extra biceps you could really swing the 15lb 338 LM to get a proper lead at running game!
Once again not down grading anyone. Physical strength and stamina is not spoken about much on here when it is definitely a contributing factor to a successful hunt. And I think you should prepare your mind and body as much as you prepare your equipment. Which if hunters did that they would be physical freaks of nature.