Shimano can be quite picky about how you have the calliper mounted in case you ever need to claim warranty. Their standard mounting position and SRAMs is ever so slightly different. I would suggest going with whatever Shimano supply / recommend for their calliper.
There isn't any definition for how stiff the connection between two mounting points needs to be, but Shimano callipers do have a habit of snapping at the mounting tabs and I have known them to point the finger at the frame or adapter.
Shimano can be quite picky about how you have the calliper mounted in case you ever need to claim warranty. Their standard mounting position and SRAMs is ever so slightly different. I would suggest going with whatever Shimano supply / recommend for their calliper.