Sorry I think I was wrong about the driver improving the performance, however the performance appears to have improved, I'm just not quite sure what caused this improvement.
To test this I rolled back to 3 previous drivers and found that my PC's performance in Horizon 3 was fairly similar across all of the drivers I tested.
I started with the 387.92 driver and then rolled back to these three drivers: 385.69, 385.41, and the 384.94 driver from July 24th.
I'm wondering if there was an update between late August and October for Horizon 3 which improved the performance, as in late August I remember trying to run the game with 4X MSAA and getting performance drops below 60 fps which no longer occur. Or perhaps these performance improvements are related to the Windows 10 Creators Update as I updated to that in September and haven't played the game much since August.