Although I hold NO truck with FPE as having anything to do with actual killing power, I will attempt some estimates and provide the results with your stats.
50gr. of 2F and a .715" ball from a 28" bl., with the probable blowby (even though the patches aren't burnt crisps) I'd estimate about 600fps.
600 x 600 x 545 (from my mould) = 196,2000,000 divided by 450240 = fpe of 435.77fpe. If it got 700fps- highly unlikely, it would generate 593.18FPE
This is about the same as a 28" .45 calibre ML rifle loaded with .445" RB and 40gr. of powder (454fpe), with an equal 50gr., 592fpe.
These stats taken from Lyman's outdated Black Powder Handbook. We know their speeds to be way out of whack with what today's powders deliver, thus the .45 would produce approx. 200fps higher speed, which would increase it's FPE to 748 - LOL.
If you generate enough FPS & Momentum to penetrate to and through the vital organs, fpe is meaningless. The formula itself is used to assign inappropriate "power" to higher velocity loads, even though of less actual killing power.
For Example - a .36 calibre round ball, driven through a 43" (Lyman's Stats) by 70gr. 3f G-0, produces 2,505fps and 988fpe. Does this load produce more killing power than a 100gr. 2F load in a Bess at 860fpe - at the muzzle? What about at 50 yards?
I'd say negative.
Due to the low initial velocity which is well below the speed of sound, the big ball will slow very little over the 50yard range. 50 yards would be still point blank - maybe 50 to 75fps slower than MV - for the .715" ball. Even if it started at 700fps, it would still be making 642fps at 50 yards and the slower it is going, the less it will lose.
The a-fore mentioned .36cal. ball will slow an incredible amount - dropping it's speed to just over 700fps, some 800fps less than it's MV according to Lyman. This drops it's FPE from 988fpe to 412fpe.
