While I now prefer roundball shooting I have shot literally thousands of rounds of saboted bullets out of at east 5 different fast twist inlines sitting on top of 90-110 grains of black powder. Ive killed about 25 whitetails with 300 grain XTPs in either Hornady or MMP sabots most of the time with black powder as the propellant. I never found the substitutes as quite as accurate. In fact Knight Rifles used to recommend real black powder or Pyrodex Select for best accuracy.
This changed a little with hotter ignitions such as 209 primers. The hotter ignitions helped the substitutes I felt in the accuracy department.

Obviously you have to clean between shots when using sabots and black powder. The best way is to just dampen a patch on your tongue, run it with short strokes down the bore , then turn it over and run it again.

When shooting sabots you must let the barrel cool between shots for best accuracy. Nothing will destroy a group faster than pushing a plastic sabot down a smoking hot barrel.

Even though I don't hunt with those rifles anymore dont be afraid to use the real black out of em. It works well.