Bucksnort1,
Something to keep in mind is that when the 357 was being developed 1930's by overloading 38 Special in a heavy frame 38 Special revolver, lead bullets are what were used. These were cast bullets with some tin added to harden them over what swaged lead is usually hardened to, but lead nonetheless. So it follows that plated bullets should be possible to drive to such velocities.
Some thick-plated bullets like the Speer Gold Dot's use actual gilding metal for plating, but because copper and high copper alloys can only be hardened by working them and not chemically or by heat treating, these plated jackets, regardless of alloy, are never as hard as jackets formed by stamping out cups from a rolled sheet and die forming them into jackets. For this reason, they behave more like hard cast bullets than anything else, offering similarly lower start pressures than jacketed bullets do. The softer surface also is abraded more easily than jackets, so copper fouling from them can build up faster when they are driven hard.
To get around that problem with its Gold Dot high power rifle bullets, Speer applies a lubricating coating of hex-form boron nitride to them (hBN). I think, if you want magnum performance from them it is a good idea to do that, too. However, there is another method you may find easier. G. David Tubb sells a hBN both for bullet coating and as a powder additive he calls
Tubb Dust. You mix about 12 grains into a pound of powder. It's extremely fine and spreads around and tends to get on your powder measure and everything else, but it effectively cuts way down on copper fouling in your bore by depositing some in the bore with every shot.
In theory, you can also do that with moly powder. The problem is that acid radicals can form from the powder if it isn't an acid-neutralized type, and they would accelerate powder aging and consume some of the powder's stabilizer over time, so I can't recommend adding it to powder until you are about to use it, and then you don't want the loads sitting around for decades. Tubb dust is chemically neutral to powder and the grade of fineness was carefully chosen to spread around well in the bore. There may be something else in it. I don't know, but it does work.
