Before I had H&R put a 44mag barrel on my SB1 I asked about using P ammo. I was told absolutely not. Obviously they have a legal team worried about liability but it was a quick and emphatic no.
This does not surprise me any. H&R does not want you to use 5.56 NATO ammo in a 223 either, even on an SB 2 frame. You can load up any ammo to unsafe levels, so when you say +P with a 44 Magnum, I would recoil at that also. What is +P for a 44 Magnum? 40K psi, 44K psi?
I would think that if a 445 SM @ 43.3K CUP is too much pressure for a SB 1/Cast Iron frame, then a 44 Magnum at just 8% less pressure, would not be all that safe either. Yet there are thousands of 44 Mags setting on SB 1/ cast iron frames, none with a problem. I say none with a problem, because if there were, they would not be putting 44 Magnum barrels on those frames.
I am still curious how the 43.3K CUP was determined and who did it. Plus a comparison of one cartridge's CUP does not correlate directly to another cartridge CUP pressure, especially if the case capacity is different.
I fully understand why anyone would say not to go into uncharted waters, because some fool could load up a case to very high pressures and then blow up a gun or worse. They would come back and say, you said it was OK to load cartridge X with my SB 1 frame. But that could happen with any frame with any case if someone uses unsafe loading practices or pushes the limit all the time.
Good Luck and Good Shooting