I've been in on half a dozen elk kills with 50's and a 54, the 50's with various conicals and the 54 with an RB. Call it coincidence or different shooting skills, but the four 50 cal conical kills all required followup shots, while the two 54 cal RB kills were one shot deals. Maybe write it off to skill, but the 54 shooter is a heck of a hunter and marksman, and made his two kills inside 50 yards, while the 50 cal shots were all over 50 yards and not as well placed.
It's tempting to say that the elk died quick because the 54 RB was more effective than the 50 conical, but as you can see that's not the most accurate interpretation.
I'd say whatever you use, hunt well and shoot straight. If you're feeling a little twitchy about your 50, it's a good thing you're shooting a Lyman. It's relatively cheap to drop in another barrel, and it would be a small thing to switch to 54 caliber for your elk hunts.
As for RB versus conical in the 54, that's a good question for lots of folks to circle the fire hydrant with a leg raised. Based on lots of experience, if you got a 54 cal fast twist GPH barrel I'd be inclined to go with the Hornady Great Plains or the home-cast Lyman equivalent. But I find the 54 cal slow twist GPR and RBs to be a lot more versatile choice. The RBs are plenty for elk at reasonable ranges while the RB is also more useful and interesting for deer and small game.
I'd happily shoot your 50 for elk if that's all I had, just for the joy of being able to hunt elk. But I'd precede the hunt with a whole lot of practice and take extreme care with the shot. I'd also look into the 50 cal Hornady Great Plains or home-cast Lyman equivalent for my elk hunting bullet. But I'd be even happier upsizing to 54 since it's so easy with the Lyman.