I want to be able to tell them "Hey, Green at 600ms will still give us an SEO boost over Green at 800ms, so we should do this," but I am having a hard time establishing whether that is actually true. Does anyone know for sure?
More broadly, prioritisation isn't about whether a piece of work will achieve its aim, it's about understanding which options are the best use of limited resources. For example, you may be able to say confidently that reducing CWV from 800ms to 600ms will increase your traffic from Google search by 5% but that's immaterial until it's compared against other options -- there's an opportunity cost associated with all work.
Personally, I would be surprised if reducing CWV from 800ms to 600ms is the best use of your resources, unless your business is one of the few that has a strong organic search strategy with organic search accounting for a meaningful volume of revenue -- nowadays, most companies find paid ads are much more effective.
As a first approximation, the difference between 600ms and 800ms will probably be similar to the difference between 800ms and 1000ms. You should be able to get a rough figure of what that difference is with 1 line of change to your code.