Thanks for this request.
We've debated this for a while, and we think we will add something here in future as it is a semi-common request.
The reason for our slowness on this, is that it doesn't feel like the right move for your users. No user is going to care if a developer thinks 10s is an acceptable load time generally.
Our experience score isn't based on what Raygun thinks is a great score either, but rather off the research done by companies like Google and others that measure user satisfaction with load times. This measures when users will be frustrated - the 'why' isn't a relevant factor in satisfaction - if it's slow, it's slow, and that's not good.
Celebrating a "Good" score where you moved the goal posts to say a long time is "good" isn't really the intention of a product that measures user experience. Fast software is good software.