Not done a great deal of testing but this seems the best yet, also using the latest NVidia drivers.
One thing I still have to do is untick CPU0 in affinity otherwise I get too much CPU load on the first core with not much happening elsewhere, (the magic of DX12 running like a DX9 game).
Before I was playing it at 4k capped at 30 and now I can actually consider going for 60fps at a lower res. i7 930 1070.
For anyone at MS, the store/game update is still fucked up. I boot the game, it notifies about the update and takes you to the store, you download it, 2gb in this case, start the game and it will tell you about the update again, you go back and it will do a 20-30mb download and that's it. You start the game and it will again tell you about the update, this cycle can go forever and the store page will have an arrow down option to redownload the whole game only.
What I do to get around this is click on games within the store, navigate to Downloads and Updates page list, then click on the game thumbnail, this will take you to the games store page but magically a blue update button will appear next to the box art, then you simply click on it and it will finalize the update properly, and the bottom right corner it will tell you Forza H 3 has been updated, check it out. Letting the game take you to the store will always bring up some partial store page and allows the infinite update cycle. Inevitably some users will just see no other option than to just download the whole game.