From what I can see, headroom relates to the density of the top wood. Denser tops, like Red spruce or Sitka, tend to have more headroom than the less dense woods like WRC and Engelmann spruce. Wood varies, of course, and it's often possible to find a piece of one species that will match another, even if they're usually quite different. I have a WRC top in my stash that has exactly the same density and stiffness as an Red spruce one: the cedar is a bit on the heavy side, and the spruce is lighter than usual. It's going to make an interesting pair.
The redwood I've seen is, in many respects, usually simply a denser version of WRC. As with all softwoods, the Young's modulus along the grain which determines the bending stiffness at a given thickness, tracks the density pretty well in most cases. Most of the redwood I've handled is in the density class with Sitka and Red spruce, with similar surface hardness. If you work the top to a given stiffness higher density one will end up a bit heavier. Light tops tend to be 'responsive' and heavier ones usually have more 'headroom' all else equal. Otherwise WRC and redwood both tend to have low damping, which gives them a similar timbre.
I'll note that on a few occasions I have run into redwood that has higher density coupled with low long-grain stiffness, and high damping: 'cardboard'. Usually this is 'salvaged' wood, from old stumps or structures such as railroad trestles. In one case the railroad wood had been 'fire proofed'. The stump wood may have been over stressed by the weight of the tree, along with a high level of the usual built-in stress, which can actually crush the wood cells near the center of the tree at the base. This is not true of all 'salvage' wood; Craig Carter's 'LS' redwood was wonderful stuff. Still, it's something for the luthier to watch for if what you're after is the real 'redwood' sound.
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