We used SS bolts to secure many tonnes of stone to buildings! So SS bolts are not very much less strong than mild steel.
They do tend to relax a little so not really the best for brakes.
Mind you SS bolts are used extensively in some Custum Cars.
That's a sweeping statement, right there.
Bolts are graded for strength.
All bolts.
You can get hi tensile stainless...and even titanium bolts.
And you can get cheap-ass ebay toffee strength bolts of unknown strength.
Rear calliper carriers are at least 10.9...maybe even 12.8. That's for a reason.
There tends not to be too much of a lever moment on tonnes of stone hanging off buildings...the stone tends to remain where you put it!
Brakes carry leverage loads and suffer heat cycling.
I would have no issues running stainless bolts if they are high tensile...but every brake securing bolt I've seen had been high tensile for a reason...and it's a brave man who would risk unknown bolts on the thing that stops you!