The copper coating on the FPB does nothing for strength, just covers the lead. The lead is a lead alloy that is tougher than pure lead, so it takes more force to fragment it, but it will fragment. After taking two cows with the FPB (one through the elbow bone) I would have no hesitation using it on a bull.

That said, sabots are probably a better choice all around. The nosler partition in 300 grain is an elk killer. Know of no one who has anything bad to say about its terminal performance. Barnes bullets are another excellent choice (either as a sabot or as a Thor).