I should have been clearer. If the factory is using those averages, as they must, to determine the 'proper' thickness for each species, then the WRC tops will be thicker then the Red spruce for any given design, but the cedar tops will still be lighter
on average.
This is due to the fact that the Young's modulus along the grain, which determines the stiffness at a given thickness, tracks the density quite well, and follows the same rule for all softwoods. The relationship there is close to linear; as the density rises so does the Young's modulus along the grain. However, bending stiffness goes as the cube of thickness: twice as thick is eight times as stiff of the Young's modulus is the same, and 25% thicker gives about twice the stiffness.
Cross grain stiffness is pretty much a function of how well quartered the top is, and drops off fast when the ring lines go a little bit off perpendicular. If you plot the long grain Young's modulus against density most of the points fall close to the same line: about 60% are within 10% plus or minus. If you do the same for cross grain stiffness, you get almost a perfect scatter: there's no discernible relationship. Cross stiffness is acoustically important to some degree, but does not seem to help much structurally, and is enough lower than long grain stiffness that it can usually be ignored in determining how thick to make the top. That's a good thing, given how variable it is.
If you have two pieces of wood, and the Red spruce is 50% denser than the WRC, the spruce will have a Young's modulus that's about twice as high. The WRC top only needs to be made 25% thicker to get up to the same stiffness, not the 50% thicker that would make them weigh the same, and should end up about 20% lighter. Most of the weight of the top is the top itself; bracing might account for about 30% of the total, and could easily be the same in both tops. so there's still a weight saving.
I'll note that when headroom is desired makers often use wood that has prominent late wood lines. The late wood adds stiffness along the grain, but it also adds weight even faster, so that sort of wood can be made thinner, but ends up weighing more at a given stiffness. The added mass helps to give the top the 'resistance' that it needs to have lots of headroom, but, again, it also cuts down on 'responsiveness'. 'Horses for courses' as the Brits say.
The bridge, and brace profile, can also be used to fine tune these things. And there is a lot of variation within each species, so, again, an individual maker can pick and choose and find wood of most species that will work to produce the desired sound. You don't seem many cedar topped guitars that were designed and built for 'headroom' for the same reason you don't see many Red spruce topped guitars that were built for 'responsiveness': it's easier to go with what works naturally. But it can be done.
See why I used the shorthand version?
