I've never seen a chart even remotely close to this for off-axis HF behaviour—and you appear to be using a conical waveguide for the tweeter which isn't all that unusual. I admit to being puzzled. What are other designers missing?
The great off-axis response/high frequency extension is due primarily to the smaller 19mm dome.
The combination of waveguide, 19mm driver, and crossover offer many improvements if taken advantage of.
This system is more complex than it appears... for example, in our implementation the diameter of the waveguide determines the corner frequency, provides time alignment, and controls directivity (amongst other things)