How much do you, the bike, your bags, food, full water bottles, clothes, etc weigh all together? 5 pounds is what percent of that? I'm guessing 2 or 3 percent. Probably not the most significant factor. Besides, 27 pounds isn't really that heavy. My Miyata 210 is about the same weight, and I've done plenty of long climbs on it. My Schwinn Varsity that I had in High School was close to 40 pounds, now that's heavy.
Ride the bike that feels the best. If you like the RS2, but are only concerned about its saddle to bar drop, why not just raise its bars? Can you put fenders on it? If not, have you ridden it much in the rain to see if you'd want to ride it for a long brevet in the rain? Do you get much rain during brevets in your area (Boston, I'd guess from your handle, which I think does mean a good chance of rain). After a few hundred kilometers of muddy water thrown up on your chest and back, you may be wishing for a heavier bike with fenders.
How are you planning on carrying gear during the brevets? Will you want a rack? If so, which bikes can you mount one on?