After using the Coos Bay T cut several times over the last 3-4 weeks, I am sold on it and don't see the need to be doing bore cuts on leaners (at least in my head). In reality, both the Coos Bay and boring are successful ways to tackle the problem of barber chairing, by addressing fundamental mechanics of materials, but the Coos Bay works in situations where you can't bore and I think it lends more control over the felling.
I think it's important to consider the fundamentals of the barber chair event. Essentially, it's caused by loads on the tree (either due to lean+gravity, mass+gravity, or wind load, among others) that create a strong shear plane within the central grain (it could also be caused by a Dutchman that acts like a splitting wedge and shears the grain, but that's a different case to consider).
The bore tackles the shear problem by removing much of the central grain. So there's significantly less of it to shear. It also removes outer holding wood in the vicinity of the typical shear plane. Then you're usually left with a back strap, far from the center so it has good leverage, holding things together until the final cut. That strap is a bit of an on/off deal -- once you get through it, the tree should go over instantly.
The Coos Bay T removes outer holding wood that acts or leverages against the central grain to create the shear, and also removes much of the central grain, only leaving a small perpendicular strap. You can cut away at that and the tree will more gradually begin to go over. There may be a mini barber-chair within the small strap, but it's been isolated to a small grain region far from the outer holding wood, so it's not of significance.
My personal experience is that the Coos Bay T is easier and more versatile given a range of tree and bar sizes you may run into, and it also slows down the final felling action for a bit more control.
I have not tried the triangle enough to want to comment on it. But again, I think the mechanics of the shear created between inner grain and outer holding wood can explain it.
