Wow, it's amazing something so simple can be so confusing.
According to my Boeing manuals, a wet runway does not need to consider braking action at all.
Only a contaminated or slippery runway must consider braking action.
This is why the performance OPT application for my Boeing aircraft asks whether the runway is:
1. Dry
2. Wet
3. Good
4. Good/Medium
5. Medium
6. Medium/Poor
7. Poor
Options 3-7 are for contaminated or slippery runways only.
So, go ahead piratepete, and use option 5 every time there's a bit of rain. It will cost you around 6000kg of payload on a typical 2250M runway.
The rest of us in the world will use option 2, as prescribed by the Boeing manuals.