"How much performance would one give up with a D style bow?"
Well, depends on what you want to hunt. There are two very dead Asiatic Buffalo in Australia that met up with my relatively light 75-pound Howard Hill longbow and two heavy harwood arrows. I could have used a heavier bow, but I went for accuracy over heavy poundage, and it paid off.
A big Asiatic buffalo can be about 350 pounds larger than a cape buffalo. The Hill bow heart shot them both, after slamming through ultra-thick hide and even thicker bone.
If you want to hunt something tougher to kill than that, say Rhino or Elephant, you could buy a "faster" bow, but a Hill bow would also get the job done, AND it would do it with the accuracy for which the Hill bows are famous.
Love them tight groups!