Good post Curl!
I agree that many late BP rifles were reproofed as full nitro guns but in using them as such, you will soon see that is not the way to go.
Part of the equation, I am sure, is the notion that at 10,000psi, the brass cartridge case is fully adhered to the barrel walls so I would guess that the strength of the barrel is the most critical aspect of the equation but not the only consideration for sure.