Since I was tagged, I will speak up. My experience with the Hydrosolid is limited. I have used them on elephant and a couple of pg. I have not shot an elephant with a brain shot, but see no reason there would be a difference to a quality solid. On a heart lung shot there does seem to be a larger wound channel than you would get from a normal solid. Since solids are used on elephants, the hydrosolid seems the way to go to me. As a side note, the skinners were puzzled by the remains of the nose that fragments at the entry point.
On buff, I would not use hydrosolids if I was hunting herds. You will probably have a pass through and the ph may not allow you the shot of a lifetime. Bachelor groups would probably be the only time I would consider useing a hydrosolid. I have never shot a charging buff, but agree with Red Leg. An A-Frame, TBBC, TSX type bullet will reach the brain, or spine if the head is already lowered. The best use of a hydrosolid would be if the buffalo runs after the first shot, which will probably happen. Then a hydrosolid would be a great raking shot and should reach the vitals through the rumen.