Mauser,
The limitations on the Model 926 (as well as the Model 925) is the strength of the barrel to frame lock-up.
MOST factory .38 S&W ammo made today is made to fairly low pressure/velocity levels due to the existence of many thousands of earlier break action/hinged frame revolvers, which can not or should not handle hotter loads.
I have fired some (a limited number of) hand loaded, specialty SD (125 gr JHP at 850 fps from 4" bbl) made by a local, small scale commercial reloader, through a M925 without any apparent ill effects. Granted this was 25 rounds and not a steady or large number of those rounds.
IN MY OPINION - those models can handle a round that is a "bit" more powerful than factory loads. If I had the capability of handloading, I would probably make a few for occasional use - but would mostly stick with factory load levels. Again, just my opinion - I don't know of any studies done on this category or type of revolver in which a tester ran loads up to destructive levels.