Terrain should be one of your main concerns, especially in the mountains. The fact of the matter is, many projectiles will kill an elk. However, that does not mean they will fall in short order. Elk can run a long way after being hit. I don't know about you but I do not like having to track or pack out elk that has ran down slope into an abyss. Been there, done that too many times.
There's also penetration issues with some lighter conical, which can directly determine the amount of blood left on the ground. On an evening hunt, especially in the mountains, right before dark would be the worse time to experience such.
Then there's shot distance to be considered. You will increase your chances much better with a heavier conical over a RB or lighter conical on longer shots. On my last elk I put a .50 Maxi Ball through an elk well past 100 yards. Elks was shot in the vitals. It took a couple steps forward and stopped behind a downed pine. It wasn't going anywhere but I reloaded and put it down with the second shot anyway. Only recovered one of the two Maxi's and it was just about to go out the offside hide. I still have that Maxi. If a 370 grain Maxi will not go through an elk from that distance, I have no confidence a lighter conical would.
Hit any kind of solid bone with a lighter concial and your chances just got reduced even more.
Hardness and conical design can make a difference as well, in terms of average penetration. Idaho Lewis and Idaho Ron have both figured that out. They, like some other hard core elk hunters out west now use conical(s) that weigh 500 grains, or close to it.
With that said, you and I both will be limited with a .54 GPR as to the heaviest and most accurate conical we can find. The slower 1:60 twist is going to be the determining factor. I purchased my GPR due to its very good accuracy with a RB. I only hunt deer now.
Good luck to you on your quest.