I have two Rivendells. A "Road" and an "All Rounder". Same size & dimensions, even built by the same guy (Joe Stark) in the same year (2001). The AR has heavier gauge tubing, but not a whole lot so. It's about seven pounds heavier due to the tubing, a rear rack, triple chainrings & mech, Brooks B17, and 40mm Conti Top Touring 2K tires on Sun CR-18 rims. The Road has downtube shifters on a compact double, ti cassette, ti WTB Deva and Pasela Tourguards (30mm actual) on Open Pro rims (28 spoke front). While the bars are the same (46cm Nitto Dirt Drops), they are about 30mm higher on the AR.
So here's the much-promised 'servatin': I went on back to back rides over two days on these bikes. First on the Road, then the next day on the AR. Same route, a 33 mile out and back that has a pretty consistent grade (THIS is approximately the ride, but a different route back). Both days I was feeling good and had a nice espresso. Priorities!
I don't normally do the time keeping thing, but this was a test. For science.
Road stats: 61 minutes to turn around spot @ 16.6 miles. Return home was 80 minutes (uphill, and tired)
AR stats: 66 minutes to turn around spot. Return was the same at 80 minutes.
Two variables to throw in. I was playing leapfrog with some cyclists on the "Road Day". Finally got tired of it and dropped down and spun past them. I kept up the speed so they wouldn't catch me. Lame, but I was tired of passing/being passed by them cuz' they wouldn't say "hi". Serious.
Second thing is that I ate a snack bar thingie at the turn around today. I may have bonked a little bit on the last little bit the previous day. So possibly my time was artificially faster going down on the Road, and possibly artificially faster returning on the AR.
The conclusion: It appears that for a couple hours of riding, in a non-competitive manner, weight doesn't matter much. Again, this was only 30+ miles, by myself and no real hills. I was just as comfortable on both bikes, which is a big factor. I sure love the way the Road feels, but am having a hard time justifying keeping them both!
A kewl aside: another roadie who didn't want to say hi had to stop and dismount to lift his delicate bike over the gate getting off the trail. I simply road around the gate on the pea gravel and kept on going. Fat tires sure are nice!