There is a difference between a GP that's been taken out of the box and to the firing line, and one that's been thoroughly prepped. The way Ruger ships them, they are full of rough surfaces and burred edges. Do "everything" to them, and I believe they should last quite a bit longer. As long as you keep them clean enough and don't do obviously abusive things to them.
Also, flame cutting is flame cutting. 125-grain .357s will do it, no matter how big and strong the parts are that they are cutting.
---------------------
ETA: Interestingly, the 4-inch Gendarmerie with a full underlug is 3 ounces lighter than a current Model 19 Classic 4-inch with a partial lug.