Not sure about the CAT C/D criteria for final approach and the numbers you quote. I fly -700 & -800, can't quote for -900. Our MLW is less than yours so we fall into C.
For a circle the FCTM quotes 2 categories. I understand that for 2 engines F15 gear down it is CAT C; SE ops where circling is at F5 it is CAT D due higher speed.
Regarding Wx minima for circling; I'm always amazed at 2400m being used. If you do the maths and calculate the turning radius for 150kts I think you'll find that the diameter is >2400m. Thus you'd lose sight of the RWY turning base. Hence a CAA in an older operation insisted we use 3600m in our Ops manual. However, in JAR OPs 2400m is legal.
It's a little the same with some NPA approaches where the required vis is far too short considering the distance you are out at MDA. They don't tie up and you'd never see the Visual reference at MDA if the vis is at minima. However, you are allowed to go and have a look. Possibly better than diverting when in fact the estimated vis was from the tower whereas you are 2-3nm away on appraoch. It might be OK out there. Same could be said for the circle. It takes good crew discipline not to push it too far. So perhaps it's not an exact science.