My experience is with a .44 mag, not a .357, but same rifle.
One thing to note is all the micro-grooved barrels I've seen slugged and mic'ed are quite a bit larger than traditionally rifled barrels meant for the same cartridges. My .44 mag barrel slugged at .4315" when normal .44 mag barrels are .429". So it's a full .0025" larger than nominal groove diameter.
I never tried Campro but Berry's plated bullets patterned like buckshot above ~900fps. The .004" copper plating is just too thin for the micro-groove rifling. Campro are twice or more thick copper and I've heard of a lot of guys having success to mid-magnum loads but not always performing with full-house magnum loads. I struggled to get cast to work in my rifle and eventually gave up and started powder coating. I tried three different molds, several different hardnesses of alloy, .001" oversized and .002" oversized, gas checked, nothing worked. They would all work with low-end .44 special loads but even a warm special or a low end magnum would end up with many hours spent scrubbing lead out of the bore and a target that looked like it was hit with buckshot from a cylinder bore. The only bullet I cast that work with magnum loads is powder-coated. I use the Lee 310gr gas-checked mold. I cast them from melted extra hard target load (water quenched they're .431" as-cast), powder coat them (brings them to around .433"), then size and apply gas checks with a .432" custom made push-through sizer. Since stumbling upon over 1k Nosler JHP's at an estate sale for barely the cost of the lead, I haven't shot much cast though.
I've talked to several other local guys who have micro-groove barrels on Marlins and they all report similar things. Some guys have good results with heat-treated cast bullets to make them extra hard but for other guys even that doesn't work. Maybe yours works well with cast, maybe it doesn't, only way to know is to try. Either way if you're going to try cast I'd at least try to find something minimum .359" if not .360" for your rifle. My guess with the Campro bullets is that the groups will remain somewhat similar as the load increases but at some critical point the groups will really open up. That would likely be the point when you're pushing the plated bullets too fast for such shallow rifling such as the micro-groove barrels have.
2400 as mentioned is a beautiful powder since it can be loaded down nicely. I still prefer H110/W296 for full-house loads with jacketed bullets and find it still gives me the best velocities. Other powders I've liked in .357 mag revolvers and .44 mag revolvers and rifles is BlueDot, HS-6, and Longshot. Those three are good for special loads as well as low-mid magnums.
