I'm running the game in OpenComposite (OpenXR) AND I'm using the OpenXR Toolkit Companion App without issue. I'm not running SteamVR at all. The key seems to be disabling Turbo mode in the toolkit to make it work.
All that said, performance is still terrible. Terrible no matter which runtime I use, or which toolkit settings I change. Closest I've got to decent performance is lowering resolution to a ridiculous 30% to 40% of my headset actual, but even that still isn't smooth. I don't think there's any settings we can tweak to make this game run properly at the moment; it's just abysmally programmed.