Due to the feedback loops, isn't 2.9 potentially significantly worse than 2.8 versus 2.8 compared to 2.7?
In general, you should rather expect negative feedback loops. For example, the warming response to increased CO2 emission is logarithmic, meaning that the more CO2 you emit, the weaker effect additional effect has, and quite significantly so. Of course, our problem is that we are emitting so much, and our emissions grow so fast, that this diminishing effect is of little consolation. The point is, negative feedback loops are much more likely than positive ones.
The question becomes, are unknown feedbacks more likely to be positive or negative? I'd lean toward negative, which is what most experts think, but definitely an unknown.