Daryl- I have shot up to 150 grains of Goex 2F in three .54's in testing, but don't have records of my groups, including some at 200 yards. My Green River Leman, which I made there in 1978 in .45 caliber and later fitted a 34" .54 Orion switch barrel to for elk hunting, gave 1998 fps with 145 grains of Goex 2F. A recent test with 120 grains of Goex 2F averaged 1875 fps.
A custom left hander I made with a 32" Green River barrel gave with 120 grains of Goex 2F 1795 fps, 145 grains gave 1919 fps, 150 grains gave 2081, 1944 (25 spread) and 1958 fps in three tests, probably with different patches and lubes.
A Track .54 Carson Hawken I built, 34" Green Mtn barrel, gave with 150 grains of Goex 2F, 1948 and 1985 fps (27 spread). I didn't think there was enough velocity gain beyond 120 grains to use more powder, but I didn't compare the accuracy.
Candle Snuffer, my shooting is at 6000 feet elevation, low humidity. You are referencing the First Edition Lyman book, for those who have the second edition with no exterior ballistics. Your 48.5" 200 yard drop is from a level barrel. Read the second line down from there that shows bullet path from a 50 yard zero to be 40.03" at 200 yards. Are you zeroed at 50 yards?
Your quote of 50 inches plus (51.15") for a .54 is total drop at 1700 fps. Mine averaged 1783 fps at about 15 feet, probably about 1800 fps true MV. That shows a total drop of 46.64", or 37.71" from a 50 yard zero. But I was two inches high at 50 yards, which raises the trajectory 8 inches at 200 yards. 37.71" minus 8 inches is about 30 inches drop, closer to my measured 27.25". The 200 yards was laser ranged. I enjoy the comments and comparisons.