I got word back on this and there's a few things at play here.
First, neither Windows Mixed Reality nor the HP Reverb G2 are officially supported by DiRT Rally 2.0, which means we're relying on however they plug into Steam VR's systems natively (in other words, there's no game code specific to WMR/G2 in the game, just Steam VR code).
That being said, after some research we came across this thread which was posted just over a week ago. That comment directs to this thread where Microsoft are aware of Windows Mixed Reality / Steam VR issues and are working with Valve to rectify them. Here is the Microsoft quote for context:
It looks like the Windows Mixed Reality subreddit is very active with several Microsoft people contributing to replies, so it'd be worth keeping an eye on this for when their fix is released.
As for DiRT Rally 2.0, Windows Mixed Reality isn't supported as mentioned before, so I'm afraid there's no fix planned on our end.
Sorry I couldn't help any further, but I hope this information is useful and that you (and other WMR players) see their issues resolved soon 😞
Microsoft is working with Valve to improve WMR performance on Steam.