If you want .357 mag. or better Power, and you do not want to lose the .357 / 9MM magazine capacity, try converting a 1911 clone that's chambered in either
9MM/.38 super to 9X23MM (brass is designed for 50,000psi max psi) 9X23MM max power loads will exceed the .357 mag. and regular production ammo while not as common as 9MM Luger, (by any stretch) in stores will easily = .357mag. revolver performance, inch for inch of barrel length which the .357 Sig cannot do (it always is a little behind the .357 mag. after you start hitting 4" and up of Bar. length but it is the nxt closest to the .357 Mag. factory loaded ammo.
I hear that if one is a re loader and one has a .38 super chambered pistol then one could just buy the starline super comp brass and reload the .38 super beyond it's design parameters, but personally I don't re load, so I'll be going this conversion route, myself.
As far as I know all you'll need is either a new conversion barrel fitted or you can just have the .38/9MM barrel fitment checked and if found to be properly fitted then you can have the smith chamber ream it, function check your piece, use 10/11 round .38S mags and then your good to Go!