Does the 357mag out of a snubbie barrel generate significantly better numbers than a .40 out of a 3-3.5" sub-compact like the Glock 27 or Ruger SR40c?
No. And I can say that having both.
Now I presume we are talking about 2 inch snub .357 magnums. If 3 inch GP100 or S&W 686 then that may change but the snub .357s, like the S&W 340SC (I have and carry that) do not generate the same ballistics as a Glock 27 .40.