Hi gp193,
Every layer in a sleeping system will affect all the other layers to different degrees and the “feel” of any combination in the surface layers is also very subjective so nobody else can really say with certainty what you will feel on any specific combination. The “feel” of a thicker wool topper on top of your mattress will be combined with the layers below it and each of them will work together and contribute their individual properties to what you feel.
A thicker wool topper will feel firmer than soft foam layers for most people because it doesn’t distribute pressure over a wider surface area as well as a foam and will also reduce the amount the latex underneath the wool topper will compress and contour to your body especially with softer foam layers. In other words it will contribute it’s own type of softness under pressure points but may make the layers below it feel firmer … especially if the foam layers are softer.
I would generally tend to avoid a material or component that you aren’t familiar with and hadn’t tested in person so you know how it “feels” unless you are willing to take the risk that you won’t like it or it has a good return policy. The “best” option would be to test a wool topper in the store over the specific mattress you have so you could feel how the combination feels to you.
When you are in doubt and can’t test a topper in person then the “best advice” would be to rely on knowledgeable people who actually make or sell what you are looking for and who will have the most knowledge and experience about how their topper may feel on various types of mattress underneath the topper (although to some degree they may be guessing if they don’t know the specifics of your mattress). There are many versions of wool toppers (just like other components) which can feel different from each other. There is a list of manufacturers who make wool toppers in many densities and thicknesses in post #3 here that would be well worth talking with if you are looking to add a wool topper to your sleeping system.
I would tend to first sleep on your mattress though for a few weeks and then decide on how you want to change it based on your actual experience on the mattress because what you feel in a few weeks may be different from what you feel now.
Phoenix