I think last time i played was 4-5 month ago. But it will try if those ini-tweaks still work this evening.
Stable? I never had problems with stability. I only had the “not visible tarmac” bug sometimes… that was very “funny”… not 
/edit: The ini-settings are already there. You only put a 1 instead of a 0 or add the RTGI line to the ini for the global graphics preset. Its not like you change the whole ini-file.
You can find the ini-files here:
Forza Motorsport\Content\media\pcfamily\renderscenarios