Hi Mr yabo.
I know you are primarily putting feelers out for the opinions and feedback of other consumers, so here are some threads that may be relevant to your question, just in case you had not yet come across them:)
This thread about a Haastens mattress with the horsehair topper
Also, an older thread that revolves around a Haastens mattress and horsehair topper.
Also, you may want to keep in mind that horsehair as a topper will tend to compress a bit over time and is a bit of a firmer topper material.
I am hoping that other subscribers see your post and chime in with additional info…
Phoenix