To your point there is only 183g between Rival and Force and the majority of that coming from the crankset not even the cassette and yeah a Rival Chain and Force chain have zero weight difference. This is why I am very for mixing all three groupsets together. While Red is pretty insanely priced, the difference between Rival and Red is a whopping 541g. Now most of that weight comes from the crankset -283g and cassette -71g. SRAM has really not been open about the weight of the new Force D2, but it's supposedly only 80g or so lighter than the D1 and that's with a power meter, so it's hard to say what the true weight is.

With this said while a Red crankset teeth aren't replaceable and it costs like $700, to loose 283g for that amount is pretty great. Some carbon wheelsets will not even shed 200g.