Hello there,
I would like to inform you about the existence of the "readme" file included with the August SlipStream GX package, you should give it a read.
One of our team members had the game running on Avcon natively with XBox controllers with no problems, so the problem doesn't exist. If you meant PS3/PS4 controller thoughYou probably have plans in the works already, but here's a request to add xinput support anyway.
there is also a document included for that too.
Thank you.