I think about 2,100fps is the limit for the 450 NE when safely run through an old double rifle. However, the Ruger #1 can handle quit a bit more pressure than the old doubles. I'm loading 115gr of H4831SC behind the Hornady 500gr RN. I can't get any more powder in the case without bulging it.
I don't know how much more the gun or the shooter could handle anyway. The gun really should have been made substantially heavier.
I saw this load on the internet...............the guy was expressing caution against eyeball damage! lol!