The smaller case capacity really boosts pressures quickly - it is called progressive powder & volume has a lot to do with it all.
Pressure doesn't always equate to added velocity. Take alook at some loading data & the +P aren't always faster.
I'd watch it & if you decide to work 'em up, please do gradually, looking for all pressure signs, etc.
Back in the olden days, I loaded up some .357 loads in .38 special cases & yes, they shot fine but blew 'bout 1/2 the primer pockets - not a good thing.
Somewhere between "real" 357 loads & 38 is where you'll end up I suspect & I'd still keep a close eye on that brass.
Oh, & I'd mark those loads very well (each cartridge, not just the box) to make sure they never get into a .38 revolver.