With all of the softwoods I've measured the stiffness along the grain at a given thickness tracks the density pretty closely, and all of them follow the same rule. One of the things that changes this is curly figure, as Ken points out. So if you can find the density you can work the redwood top to the same thickness as a spruce top with the same density, and be pretty confident. Of course, there is some variation, and there's no substitute for actually measuring.
All of the redwood I've tested has been on the denser side; more like Sitka or Red ('Adirondack') spruce than Engelmann, but, again, all woods vary.
One thing that most of the redwood samples share is a low damping factor. In that respect it's like WRC, and sometimes even better. That's what produces the long, musical tap tone. It pairs well with a back material, such as Walnut, that has higher damping.
I'll note that, as with WRC, redwood has low peel strength. It tends to split easily, and you have to be very careful gluing on the bridge: make sure to do a light scrape on the top and the bridge bottom just before applying the glue to raise the 'surface energy'. I like to make the bridge wider on cedar and redwood tops: deeper along the line of string pull. This reduces the maximum stress along the back edge of the glue joint. I like to use walnut bridges for these, as it allows for the bigger footprint without adding weight. Also, if you finish the top before gluing on the bridge be very careful not to cut into the wood as you scribe the finish, especially along the back edge of the bridge.
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