I agree with MW Surveyor about the standard SP101 sights being regulated for 158 grain loads, as 125's shot quite low in my 3" version. I had to reduce the front sight height considerably to get 125 loads to point of aim. But it was worth it.
Out of my 3" SP101..................
Remington 125 grain SJHP .357 magnum 55 deg day (avg 5 shots) 1322 fps - 488 ft/lbs ME.
Remington 125 grain SJHP .357 magnum 90 deg day (avg 5 shots) 1374 fps - 527 ft/lbs ME.
NOTE that the above 2 loads are the same, just shot on days with a different ambient temperature.
Speer 125 grain GDHP .357 magnum 55 deg day (avg 5 shots) 1333 fps - 496 ft/lbs ME.
As mentioned, I suggest practicing reloading. I gave up speedloaders in favor of speed strips as they carry so much easier for me. There are some excellent YouTube videos of reloads using speed strips.