Rhino Bullets have been making 380 gr bullets for the 375. I think they're the only ones that make one that heavy. I'm not sure but didn't Woodleigh make some heavy 375 bullets also?

The theory is nice but the H&H case is of limited capacity especially with the heavy bullets. Even with the standard 300 gr. bullets, I could never come up with the published amount of powder for certain loads when trying to find that one magic load with the best performance. I forget the amount of 4350 but I could never get much powder into the case without a crush fit and an overly compressed load.