I don't ride disc brakes but I don't really understand why a lot of people act like disc brakes are stupid and rim brakes are the greatest thing out there and can not be improved upon. It kind of seems more logical to me to use a disc seperate to your wheel for braking (you know like cars have been doing for ages) as opposed to using the actual rim for braking. You are pretty much intentionally wearing out a structural part, seems illogical to me. And up until a few years ago it was impossible to ride full carbon clinchers because the heat management problem hadn't been solved yet. With disc brakes, that was never even a problem. You can ride your nice carbon clincher rims as much as you want and can because there is no heat build up in the rim and you're not even wearing out the carbon brake surface. But then again I'm not an engineer, so this may all turn out to be idiotic ramblings.
I'm so on the fence with disk brakes for road bikes.
On one hand I don't want the extra expense, cost, complication, any brake rub and maintenance (I have 10 years on maintaining motorcycle and MTB disk brakes). But then the wet braking and how fast you can wear out your rims pushes me back to disks. Maybe N+1, keep a shallow wheeled rim brake climbing bike and have a disk aero bike use each half as much and save some wear and tear.
I got a $1400 dollar low end carbon bike (Fuji SL 3.3) down to under 7.5 KG by changing out the wheelset, tires, seat, post and a few other parts. I seriously doubt this would ever be possible on a disk brake bike. 7.5 KG are we talking $5000?
Back to the subject. As has been mentioned the rotors are an area to look at. Often the OEM bars, stem, seat and seat posts have some weight to save, up to 50g a part sometimes more.
Also the wheels and tires. If there are tubes and tires, You could save some weight with Continental GP4000S2 and latex tubes or Schwalbe Pro One with Orange sealant. You just need time to sit down, remove each part, weight it and decide if the cost is worth it.