Those are good questions. I think you will do just fine with a 270 grain TSX on cape buff. I prefer a solid to an expanding bullet for followup, because the angle of the shot may be such that the bullet will have to penetrate a lot of buff before it gets to the vitals. That said, the Barnes TSX has an excellent reputation for penetration.
If you do decide to use a 300 grain solid for followup, in testing, it should group fairly close to where the 270 grain bullet does before I would use it. If it does not, simply use a 300 grain Barnes TSX for the first shot and 300 grain Barnes solids for followup shots.
