> By your logic then, adding another 20 pounds to the OP's load would also be an "insignificant" difference.<
That is true, according to the physics. But adding mass is not the topic. It should be obvious to anyone who has had high school or college physics your 20 pounds of additional mass will require more work, more effort, to get moving and to keep it moving. When discussing vertical movement, up or down, adding mass changes the equations and, of course, changes the handling of one's rig, especially when accelerating downhill. Your post, which is purely anecdotal, indicates you might have packed your mass incorrectly and contributed to your difficulty handling your bike at speed--another excellent reason for reducing one's traveling kit.
going ultralight was a delightful revelation for me. There are only four places one can make significant weight savings: tent, sleeping bag, backpack (panniers), and kitchen. Everything else--clothing, toys, furniture, toiletries--is trivial until you start adding up the savings. With a bivvy, one-pound down bag, ripstop shoulder bag, and an alcohol stove I started hacking at the other stuff. An ounce or five here and there added up to another ten pounds of stuff I did not really need to carry. It was liberating: setting up and striking camp required only a few minutes and I didn't have anything to keep track of or lose. Rampaging boredom was an issue.
From my days of ultralight backpacking, I know, anecdotally, reducing the mass on my back from 45-60 pounds to 15-20 pounds was not trivial; I literally ran up into Idaho's White Cloud Mountains instead of slogging along and hating every step of the climb. I also know, equally anecdotally, reducing my mass a mere 5 pounds was utterly insignificant; I was just as sick and tired of hauling my stupid 40-pound pack as I was hauling 50-60 pounds.
New issue of Adventure Cyclist magazine arrived while i was on Ride Idaho. There is an article on exactly this topic showing the objective (if unverifiable) analysis of different masses on the time required to complete a known climb.
david boise ID