When Colt was working on the gun that would become the Cobra, they made an experimental prototype chambered in .357 Magnum. Now in those days the .357 was really a Magnum, and not what is generally put out now. They put something like 3000 rounds through the gun while watching for the frame to stretch. It didn't. The people test firing it gave in before the gun did.
On the other hand I once examined a .38 Special Cobra that the owner had unwisely had rechambered to .357 Magnum. I have no idea what the round count was, but the gun was a mess. It ended up being scrapped. He tried to send it back to the factory for repair, but Colt would have nothing too do with it.
Occasional use of Plus P .38 Special ammunition will not hurt the revolver, but more extensive use will put it out of time in quick order. Then it's back to the factory for a tune-up and a kick-in-the-butt repair bill.
Some S&W and Taurus lightweight snubbies are rated for Plus-P ammunition. I would personally stick with the Cobra with appropriate ammunition, but those who feel hopelessly underguned unless they have the hottest available loads should stick to revolvers that are rated to use them.