How did you get the Rally Expansion downloaded? Like many others I bought/downloaded the Season Pass but in-game cannot access the server to download the expansion and outside of the game on the store page it says the expansion is unavailable.
I didn't buy the season pass; only the rally expansion.
Here is Digital Foundry with their comparison of Forza Horizon on back compat V 360.
https://youtu.be/MIpf_4-WZt4
Just as I assumed: It's perfect.