> Same ballpark, maybe, but I'd have thought native will always be easier for browser vendors to optimize
It will be. And things like the CSS Paint API are based on APIs that are poor matches for GPUs (but are easy for browsers of today to implement). I expect that those who are looking for Houdini for performance reasons will end up quite disappointed.
More broadly, I have seen the "replace the native implementation with a JavaScript implementation for better performance" idea tried many times, and I can't think of a single time it's actually worked.