mgulino,
The answer to your question can be unbelievably complicated.
First, Mr. O'Heir is correct. You've misread or miscopied the .38 Special data. 4.3 to 4.6 grains is the load range at Hodgdon for that cartridge.
The main worry about the ring is not carbon, it's lead. If you shoot lead bullets, especially with quick powders, there is a tendency to gas cut the edges of the lead bullet base and have it splatter and build up in the end of the chamber. Indeed, you can get this even in .38 Special chambers. Back when revolvers were still common for conventional pistol matches, the armorers at Camp Perry all had special cleaning reamers just for getting the lead out of K-38's and other .38 Special wadcutter revolvers. You can still buy those reamers. In a .357 chamber, the lead from .38 WC's can prevent not only quick chambering from a speed loader, but can prevent the mouth of a firmly inserted .357 from letting go of a bullet easily, raising pressure.
For jacketed bullets, the metal fouling build-up should not be an issue. Just carbon, and the big secret with carbon, if there is one, is to get solvent on it when it is still warm and not yet had time to harden. When you finish a range session, and before you put the gun away, use a small pump sprayer of Gunzilla or Bore Tech C4, then the carbon will mostly just patch out after you get it home. Keep it in a plastic bag in your gun case for the ride home, because you can get carbon-saturated liquid dripping out.
But if you are running into ejection problems due to carbon right there at the range, then sticking with .357 cases is best. I admit that is what I do.
So, then, using .38 formulas in .357 Magnum. In theory, because the .357 case is longer, it should lower pressure. The Hodgdon site used to have 231/HP38 loads for 148-grain wadcutters in these two cartridges. The bigger case was getting more pressure from a lower charge. I suspect that because these bullets are seated deeply and leave very little powder space, the primer was unseating the bullet before the powder burn got fully underway in the shorter Special case. It's so unusual, though, I asked Hodgdon to confirm it was what they had on file from their ballistic tech, and it was. Now they've changed the listing to include only Tightgroup for 148's, and the load is identical because both are seated to the length of a .357 case (not the way they are made commercially, nor would it feed in a model 52, so I don't know what inspired that move). But the old data showed some odd things can happen.
In the main, though, if you use the same crimp cannelure for the same bullet in both cartridges¹, you will get lower pressure from the larger case. QuickLOAD suggests that when you seat your 125-grain XTP in the longer .357 Mag case, you will need to go from 4.3 to 4.6 grains to match velocity (8" barrel) and to 4.9 grains of powder to make the peak pressure match (but velocity will be higher because the greater total gas quantity will raise pressure seen further down the barrel and at the muzzle, so there is more late-barrel acceleration).
¹ Some bullets have two cannelures to hit the nominal SAAMI maximum COL's for both cartridges, the lower one being used in .38 Special and the upper one with .357 Mag. This is because a .38 Special cartridge reaches SAAMI maximum COL with a bullet sticking out 0.395" beyond the case mouth, while .357 Magnum reaches SAAMI maximum COL with a bullet sticking out just 0.300" beyond the case mouth. When you use a 38 Special cannelure or crimp groove in the 357, it comes out 0.095" longer than SAAMI maximum COL, though many .357 revolvers, like Ruger, have a cylinder long enough (1.685" or longer) to accommodate that greater COL, letting you use a bit more powder and get a bit more velocity out of your loads for the same peak pressure. There is a similar relationship involving two-cannelure and two-crimp groove bullets for .44 Rem Mag and Ruger Redhawks and a number of other revolvers have enough cylinder length to accommodate extra long cartridges made with the Special crimp groove and the magnum case.