That's quite right. Weight is an advantage downhill. Extreme example is tandem. The cx of one rider and the weight of two. Ever tried to keep up with a freewheeling tandem on a downhill ? You kept up ? It was a daydream. The heavier weight is faster downhill. On moderate uphills which don't require getting out of the saddle, a steel bike will keep up momentum beter than a carbon bike. The pedalling technique required for getting the most out of this is different. Pedalling cadence has risen a load in recent years. Suited to carbon lightweights and indexed shifting. The indexing allows you to shift seamlessly and keep up the cadence. Without indexing (steel), the modulation should come more from your legs, not the drivetrain being used like a gearbox (CF). Even ultralite wheelweight at circumference (rim and tire) isn't always best. There is thing called momentum, and if you look at some of the track record attempts over long distances, you'll see guys now trying (Japan) really heavy rims.

Anyway, give it a try. Be nice to know how it goes