Typically you can save about 1/4-lb by removing the quill and bolt that goes down into the steerer tube when going from threaded to threadless. Also clamping on the outside of the steerer gives you a larger diameter interface with more contact surface-area, leading to a stiffer joint with less flex. Try flexing the bars by standing facing the bike with your legs straddling the front-tyre. Grab both bar-ends and yank one up and push the other down. With a traditional 100mm Cinelli-type quill-stem, you'll be able to flex the bar-ends 2" up and down. With the stiffer threadless design, this flex will be about 1/2 of that or just 1".