I have been thinking about this myself recently, as I return to cycling after a long, long break. I have recently purchased a handle bar harness and a Viscacha seat post bag from Revelate. I haven't used them much yet, but they seem superbly designed and made. At first they reminded me a little of my old Dana Designs rucksack in terms of seeming over built.
The Harness weighs 18oz with a large 20L dry sack. The Viscacha is 14oz and a max volume of 20L as per BPL review, so we have a total volume of 40L for a weight of 2lbs. A 40 L Porter pack from HMG weighs 26 oz, so the difference in not huge. There is probably a bit of weight saving to be had, but personally I am happy with the weight of my Revelate bags becasue:
1. Bike packs seem to me to take a lot of punishment in terms of rubbing against metal and the vibrations of riding. I don't want my seat post bag to fail when I am 8 hours ride from the road end. I also want them to be well secured or the effect on handling will be heightened.
2. My bike (Surly Ogre)is designed for durability over weight savings and any frame bag savings are going to be minor as proportion of total weight.
3. It seems to me that total weight is not as important in biking as it is in backpacking, becasue in biking the weight isn't carried on the body. I also feel that fitness and conditioning makes a huge difference in cycling.
4. I am already using UL camp, clothing and cook kit.
Bikepacking may well develop an ultralight arm in terms of bikes and bags, but I think the advantages will be less clear cut and the trade off greater than for
Chris Zimmer made Doug some Cuben panniers if I remember rightly.