The 450 GNR is nothing but a 450 Alaskan/450-348 Ackley Improved. It would be no better or worse. I have a 450-348 Ackley Imp. and it is a good cartridge.
The only thing with it is the case head is larger in diameter than the standard belted mag and pressures need to be kept lower because of the backthrust on the Encore. This defeats the powder capacity.
My 458 Win Mag will shove a 400 gr bullet to around 2200 with the powder suited for it. This is from a 14 inch barrel.
This is all a moot point since anything can be killed quite well with a 400 gr bullet at no more than 1400 fps, and generally quite a bit less, and a Contender will do that in 45/70 with a 14 inch barrel with an easy load, cheaper cases, etc..
Only people a few clicks off like myself own or shoot guns that are in this power range but it is just to have something not every shooter has not because we believe we need the power. It is just our own warped sense of fun.