My lightest, but not by design, classic steel was a Simoncini SLX with Veloce and Vento wheels for 20.4 lbs, and I'm sure I could have gotten it under 20 with a carbon seatpost, Ti chain, and Ti/Scandium cassette. (Sold it). My De Rosa Professional SLX with 9-sp DA (DT shifters) and Mavic Ksyrium Elites is 20.4 lbs, but could go with a short-cage RD and smaller range cassette, plus a much lighter saddle than the SSM Regal clone it has, and easily be under 20. It wasn't relevant to me to go there, but it is entirely possible.

My modern steel bike is well under 16.5 lbs, and my two modern carbons are well under that. I generally use lighter parts on them, if possible, but all three could easily go lower. There are tons of lighter parts out there, but I didn't build them to be light; they just are.

Cervelo's own tests showed a huge improvement with 5 lbs of lower bike weight: 36 seconds over a 71-minute course, a whopping .8%, or basically, an extra drink from your water bottle.

FWIW: I built, on purpose, a 15.1 lb 2000 Kestrel 200SCi when the UCI limit was just that. I rode it 35 miles and felt like running it over with my car. It had more carbon than a coal furnace, and rode like a piece of plywood. I sold it to someone who wanted one like that. Never again, though I kind of wish I'd kept the wheels....

Light bikes sell, and are being developed, because no one wants to have the rider next to them to have an advantage, real or percieved. The TdF pros will tell you the weight savings long ago reached the point of diminishing returns; it's about pushing through the wind, being comfortable, and being fit enough to enact the proper tactics at the proper time. For climbing, it's Power/Kg. Advancements in performance are in handling, efficiency, and the next big step is disc braking, then eventually, ABS, so all a rider has to do is squeeze the brakes and they're modulated automatically. We will see descents that approach tire adhesion limits.

Writers in bike magazines cheer for stiffness, decry "excess" flexibility in frames, and are parroted by the fools in the 41 who think they can actually be faster on the lightest, stiffest frames. Maybe they are, but I'm not sure who they are racing, or if they're actually winning podiums anywhere but in their minds. If they are racing Walter Mitty, they have to understand he always wins. In my perfect world, you draft those fools all day on your steel bike, and then outsprint them at the end, not because you have a stiffer bike, but because you have the legs and the mind to do it. Oh, wait, it happens a lot in the real world, as well. Imagine that.

I have significant upper body strength for a rider my size, just a product of my genetics and history on the farm and the mat. Yes, I feel the slight flex in Cinelli stems and bars, perhaps the bottom bracket, of steel frames, from an Ironman to a Corsa Extra. That is exactly what I want in those frames, the smoothness and predictability built in by a designer who rides and a builder who cares, and a type of frame construction that they've successfully predicted would sell for 30-40 years, over and over again.

Sure, there are lighter C&V bikes, like the carbon Ironman, but it was so flexible as to be alarming; not always knowing if the rear was still attached to the front! Sure, there are lighter and stiffer C&V bikes, like the Centurion Facet, but the buzz through the bars was very noticeable, to the point of distraction. In both cases, I prefer the steel offerings; they are abundant, and affordable, and the craftsmanship is very good. The weight is not an issue, but if a person can get one of those fine frames outfitted to be less than 20 lbs, more power to him/her, for whatever reason. It's obviously a good conversation topic.

A typical group ride in my area finds us at the parking lot, one leg over the bikes, chatting up until the last guy gets there, or someone has the intiative to start rolling. "What does that weigh?" is often the first question, so you get that over with and then roll. The topic is generally forgotten within the first mile or two. If it comes up in others' minds on the first hill, I'm never sure, but I doubt it. The climbers climb, we wait, and the caravan continues.