Hi walkpdp,
A down alternative topper and a featherbed will be fairly similar.
A featherbed or down alternative topper can add some “cush” or “surface feel” to a mattress but they compress so easily that they primarily add a little bit of cushioning directly under pressure points (like hips and shoulders) but they aren’t as effective as a foam topper (memory foam, polyfoam, or latex foam) at redistributing body weight across the surface of a sleeping system and relieving pressure across the larger surface area of the body or providing support under the more recessed areas of the body (such as the small of the back for back sleepers or the waist for side sleepers). They are also a higher maintenance item because they need to be fluffed up on a regular basis because they pack down and compress with use unlike a more resilient foam topper. There is more about featherbeds or fiberbeds compared to foam toppers in post #2 here and the other posts it links to.
There are also different types of softness/firmness that different people are more sensitive to and prefer to different degrees (see post #15 here) and a featherbed would affect the “feel” sense of softness and provide some padding and relief to specific pressure points and foam will provide more effective “pressure relief” softness.
There is more information about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support” and “pressure relief” and “feel”.
There is also more information about choosing a topper in post #2 here and the topper guidelines it links to that can help you use your sleeping experience as a reference point and guideline to help you choose the type, thickness, and firmness for a topper that has the least possible risk and the best chance for success. It also includes a link to a list of some of the better and more reliable online sources for toppers I’m aware of as well.
On a mattress that is much too firm I think it would probably be safe to say that most people would be better off and probably prefer a foam topper (either memory foam, polyfoam, or latex foam).
Phoenix