At what hardness point does a cast boolit become harder than a jacketed bullet?
Copper is around 35 BHN,LaserCast bullet are around 24 BHN there probably some of the hardest commercial cast bullet available. I used to shoot LaserCast bullets in my 357 Mag. revolvers many years ago before I started casting my own at 24 BHN you can push them to Max velocity as long as they fit properly and get no leading at all. Most of my cast handgun and rifle bullets are much softer than 24 BHN. A hard cast bullet is neither good or bad when used for it's intended purpose you just need to match the alloy hardness so that it preform as intended with the pressure and velocity your load generates.
At some point can these cast ones be rougher on the barrel than jacketed?
Not really, dirty gritty alloy might rough up your bore some but for the sake of argument lead bullet want wear out your bore. Throat erosion is the main enemy of any firearm a firearm is only going to last having a certain number of pound of powder fired through it. Burning all that powder in large quantities and in rapid succession without allow it to cool down in between shooting secessions will wear out the throat much faster than spreading all those shots out in smaller loads and over a longer period of time.
Now that I look at these 350 grainers, they do seem pretty hard. I have found them on the ground after shooting at 1200fps into a dirt bank and they were only scuffed, not deformed.
A 24 BHN bullet is pretty darn hard as cast bullets go,mine generally run for 8 to 16 BHN the only bullets I cast that's remotely that hard are for my semi-auto rifles mainly for feeding purposes through the action and to help eliminate any gas port leading. 1200 fps. is plinking load and mid range 357 load velocity in my loads,if your bullet is 24 BHN your not going to get any bullet deformation unless it hits something harder than itself.
Dirt is just going to scuff it up a bit. I recover many of my plinker and mid range 357 load bullets from the dirt berm to remelt and shoot another day. There BHN is around 12 and the noses are usually mushroomed or folded back to some extent.