@Timpo raises some really valid points about rotational weight and the difference between weight near the center of the wheel versus weight at the outer perimeter. Toward that end I find that carbon rims shave very little weight as compared with good alloy rims (commonly estimated at $5/gram, especially on a cost weighted basis. Obviously going tubeless or using Tubolito light weight tubes save far more weight per dollar, around 70-80 grams for a $35 tubolito grams. In fact Tubolitos save as much, if no slightly more, then tubless when considering the weigh of sealant required, especially in wider tires.
An Item I am installing on my Topstone Neo Carbon 3 that will be adding56 grams but should more than make up for it in performance is an Onyx rear hub. Trek put an Onyx hub on my Allant 9.9s to replace the faulty Shimano hub that it came with. I am now forever spoiled and want one on every bike I own.
Onyx hubs use a sprag clutch mechanism instead of a traditional gear & sprung pawl. Not only to they offer instant engagement (no lag when resuming pedaling) but the also completely disengage when pedaling ceases...the clutch mechanism offers zero resistance. They also use ceramic bearings which are significantly smoother and faster rolling than metal ball bearings. As the added weight is right next to the axle, the additional 100-150 grams will not be significant rotational weight (it is added unspring weight that can make a difference on a full suspension bike however). So while they add grams, they should increase speed through better bearings and complete disengagement of the clutch mechanism. The instant engagement adds a real advantage while climbing steep gravel trails too. Oh and another distinction of Onyx hubs is they go completely silent when coasting, no buzz or whirr associated with a gear spinning by sprung pawls. Oh and they can be disassembled for service in less than two minutes. They are costly but are a terrific product that enhances the ride experience any bike and made in Minnesota
Onyx Racing Hubs are quietly clutching a share of the high-end hub market.
www.bikemag.com