The following list runs starts at how easy it is to install yourself:
Optimistically, all four could drop ~450 grams/1 lb.
Measure Twice, Cut Once
If you are doing this entirely on your own, be extra careful about measurements. If you haven’t done this stuff before, know that there can be imperfections in sizing. I’ve seen issues with seatposts because either the seatpost diameter was a millimeter too small, or the frame diameter is slightly bigger. Stem length is something I’d discuss with a bike fitter, but it’s possible you’ll find out you’re on a poorly sized bike to begin with.
Back in the day carbon failure was a more likely possibility. Carbon failure can mean a very sharp edge, and all-or-nothing operation. If alloy/aluminium fails, you can still ride on it to get home. Carbon in theory should dampen shock/road noise, but it’s not really noticeable in my experience – the frame and/or fork makes a bigger difference. Depending on your frame, some metal on metal combinations can bond together over time – ask about it if concerned.
My research based on weight suggests there’s no difference between metal and carbon – the lightest stem is still around 100 grams, lightest seatpost comes in around 150 grams. However the lighter handlebars are typically carbon.
A big part about saddles is your seat bones, and everyone is different.
Saddle angle can be a personal preference. I’ve ridden some setups where it felt like I was going to fall off the back. The rational behind the angle I’ve heard from some is that nose down is more aggressive, meant for sprinting/etc while tipped back is better for climbing. But I know some who disagree it matters at all. One benefit of having the nose down was that my bib/cycling shorts were less likely to catch & tear.
Saddle rails – how the saddle anchors to the seatpost – can impact seatpost choice. Some stuff is proprietary (Ritchey stuff recently), and carbon rails had clearance requirements.
Depending on the length of seatpost, you can trim it to save weight. That said, this is not something I’d try with a carbon seatpost and more importantly – it voids warranty, in addition to limiting resale potential.
You’ve Done All That, Now What?
Depending on what’s currently on the bike, you could consider getting a higher-end cassette (gears on the backend). You can swap these yourself, but you need a chainwhip tool. Just using Shimano as an example, going from a 105 cassette to a Dura-Ace one will save you ~100 grams/3.5 ounces. That’s also wheel weight, which will help with acceleration. The trade-off is both cost, and wearability. Lighter things don’t tend to take abuse as well as others – it’s worth buying a chain length tool to replace worn chains ASAP to minimize cassette wear.
I ran a Dura-Ace chain for a while. It was light, but seemed to stretch very quickly and was costly to maintain. I’ve had less issues with an Ultegra chain. The consensus I’ve encountered was that few could really tell the difference in chain quality – they all seemed to wear out the same. Which reminds me – I should check my chain…
The rear derailleur? For the hassle, I opt for a mechanic to swap these. The weight savings is less than for a cassette – 75 grams/2.6 ounces. I think you’d notice more if the shifters were upgraded than the shifter.
The crank can be a good place to save weight. You have to be aware of the bolt pattern for chainrings – the BCD might only support compact (50/34 max) or the usual 53/39. Some cranks are setup to support a wide variety of chainrings. The next issue is bottom bracket. There’s numerous “standards”, and some are proprietary (Cannondale?). As for crank length, there’s no benefit from having longer cranks. In reality, it’s more material so more weight, and can mean less clearance when cornering with the crank on the inside of the corner down (IE if you pedal while cornering). But you can easily save 200 grams/7 ounces, and stiffer cranks means better power transfer – you’re not loosing power as the crank flex under load.
Wheels can easily be one of the most expensive means of dropping weight on a bike. I do not recommend pre-packaged wheelsets. Custom wheels are likely to cost less and get you a superior wheelset. Talk to your local bike shop, but if you’re really wanting to save money – learn to build the wheel yourself. Worst case scenario, take the wheel(s) into a bike shop to be trued/dished. Lacing a wheel is not that difficult. Rim brake wheels can get under 1 Kg/2.2 lbs in the ultra high-end category, to date the lightest disc brake wheelset I’ve encountered might be under 1.4 Kg/3 lbs. It can be a very noticable difference, going to a lighter wheelset. Less weight to power, and faster acceleration.
SRAM, Campagnolo, Electric Shifting (Di2, etc)
I haven’t ridden SRAM, but the reputation is that the offering is substantially lighter than Shimano but short-lived. But I prefer the cranks to Shimano offerings, especially since the move to four arms.
I have ridden Campy, but not extensively. I liked the thumb shifters. By reputation it’s expensive, and can be difficult to source parts locally in a pinch but that’s likely to be better in more major cities than where I am. Unlike Shimano, Campy stuff can be rebuilt – the shifters anyway.
I haven’t ridden on electric shifting, but accounts the Shimano stuff is fantastic. Automatic trimming so you’re not adjusting either derailleur. If the battery dies, it stays in the current gear. I was last told that Di2 is 30 grams lighter than the mechanical alternative. The shifter pods for triathlon/time trial (TT) are pretty awesome too.