I have used the same rifle in .54 caliber to take dikdik, warthog, kudu, and eland.
The best regulating load in that particular rifle was 100 grains of FFg with a 460 grain Hornady Great Plains flat nose. Both barrels would print 4 inches apart at 100 yards which is my self imposed limit with a muzzleloader. This was done when there were several companies producing full bore lead conical bullets. I tried every combination that I could find and even though I would have preferred a heavier powder charge this was by far the most accurate.
With a full bore conical a soft alloy or pure lead is the only material that a bullet can be made of and still be able to push it from the muzzle. Weight retention and expansion will be very good usually at the sacrifice of penetration. The combination of 100 grains of FFg and that 460 grain bullet was found in the middle of the heart of an eland shot at a range of 50 yards. We had a dead eland but not enough penetration IMHO. I have not taken a wildebeest with a muzzleloader but my instinct would be to go for a double lung and avoid the shoulder.
I have used a 300 gr. hard cast .50 caliber bullet in a .54 caliber sabot on top of a 130 grains of FFg and can tell you that combo out penetrates by a significant margin the heavier soft lead conical bullets.
If using a traditional conical or maxi-ball in a double rifle the heaviest bullet and powder charge that gives satisfactory regulation is the way to go. The max charge rating of the manufacturer and rate of twist are the limiting factors.
If I can find them I will post pics of some of the conicals I have recovered from animals over the years.