Hi felice,
The down alternative mattress pads (or fiberbeds) are made from polyester fiber or in some cases (like ingeo) bioplastics and will really only help with surface feel and are not really resilient enough to provide a lot of extra cushioning at least in the long term. They will compress fairly quickly and lose their loft. High quality down will be more durable and more “fluffable” but of course can also have it’s own issues with allergies and good quality (without the feathers that will stick through) is also more expensive. It also is not as resilient as wool and is more sensitive to moisture and dust mites. Synthetic fibers are also not as breathable and temperature regulating or resilient as wool or silk.
Personally I would go in the direction of wool or other natural natural fibers like silk in spite of the greater expense if you still need to do some fine tuning after you have slept on the mattress for a few weeks. A down alternative will give you some localized cushioning for a while but not to the same degree as wool and it certainly won’t last as long. You could also look at a shredded latex topper such as this or this. One of our members “coventry” ordered the Amazon version (see post #7 here) and hopefully she will have a chance to give us her feedback.
ADDED: You can see her review here.
The biggest advantage of a fiberbed is that they are good value and low cost so they are inexpensive to try without a lot of risk. If they work for you then you have an inexpensive solution for a while.
You also made a good choice IMO in going a little bit firmer rather than softer because it is much easier to soften up a mattress than it is to make it firmer 
Phoenix