You do lose some of the precision compared to using the fine crosshairs, but the fiber optic worked much better for me at dusk. I was able to achieve approximately a 4in three shot grouping with it at 100yds. I was pretty happy with that result.
The MR-X is also my first muzzleloader. What worked for me was an Eabco peeprib and Williams fiber optic peep sight.
Eabco peeprib:
https://www.amazon.com/EABCO-Peeprib-Optima-Kodiak-Accura/dp/B011IWOIUG
Williams sight...