Very well done, Openshaw!
I gave your horizons a try and like them a lot - actually I have been waiting for this!
I used TheWitcher's mod for original M&B and always thought it to be an enrichment.
Since I am using Polished Landscapes, which also modifies terrain_borders, it doesn't blend in perfectly with your fine horizons for me. I uncommented the terrain_borders entry from PL in the module.ini and let it load yours instead, but there are some aesthetic issues. It works though.
Although I will not use your horizons together with PL because of the aforementioned, I have to state again that I do like your work very much and thank you for sharing it!
If you ever take a shot at making your horizons aesthetically compatible with PL - I'd be the first to install it!