The weight savings is pretty small with special fasteners, but it can do things for the rider mentally. The mental boost of riding your tricked out bike can make you ride/try harder.
I've run just 3 rotor screws on my Santa Cruz for 15+ years Saves 50% of the weight! Used nylon screws for bottle cages but replaced them annually. Learned to do that after having them break after about 2 years of use and losing the bottle and cage. Would not use aluminum fasteners on the stem or seatpost but no problem on things like derailleur or brake mounting (Hayes or "post" style only) or shifter. Best boost to the ride though is lighter wheels and tubeless tires.
Lightweight fasteners are all over my Ducati. Standard fare. Most are steel of some type, but almost all have some kind of reduced head. Imagine like a button head socket cap screw but with the radius of the button inverted. Another style is a reduced hex size but with a flange under the hex so that the bearing surface area remains the same. Again, not a big savings, but every little bit helps when weight control is part of the design requirement. It's why even the kickstand is forged aluminum.