Personally, I think that the toughness of the cape buff is a bit over played. I initially used solids (they happened to be Barnes). They punched right through (killed an ox-pecker on the off side--just not his day), but after analyzing the results, I have changed to a Barnes X type bullet. I know that they don't have the original X anymore, that's why I said X type. I'm still using the original X's because I bought a bunch of them, but their newer models are probably even better. I think it is more important to make sure that you have a good shot, are not too distant, and place your shot(s) well. Of course you must use a strong bullet, not a typical lead softnose. Most PH's will be picky and selective about the desirable range and position of the buff; of course shot placement is up to you. Be certain you are up to the task, i.e. practice. While I think a .416 is about perfect, I don't have one and have always had to use a smaller caliber, but that's because I don't have a .416. I guess what I'm saying is don't make more out of it than it is. A good Barnes X type will do the job just great, provided you do your part. No matter the bullet (or caliber), neither will make up for a poorly placed shot.
The first PH I was with said that he didn't allow shots on buff over 75 yards max, but strongly preferred less, indeed he preferred and worked for 50 or less, and never allowed an initial shot at buff after 4:00 PM. I consider that good advice, and whether other PH's enforced such or not, have stuck to it. And lest someone takes me wrong, I'm not saying buff aren't tough guys, just maybeso not quite what they are played up to be. Again, the key is good shot placement--don't get their adrenaline ginning with a poor shot!
Good luck.