if the second plot is for the 420 the small difference in distortion is quite surprising Strange
personally i would choose 150 plus one sub per side any day
You can't extrapolate a distortion measurement at one SPL to a higher one because distortion is nonlinear, it doesn't increase proportionally. When a speaker nears its limits, it skyrockets very quickly. So if 96dB is not the limit of the speaker, the distortion levels tell you very little about how loud it can actually play.
Fortunately in this case we have apples-to-apples S&R SPL tests for both speakers, and as you can see, the KH420 plays about 6-7dB louder than the 150 below 100hz, and above that the margin grows to as much as 14dB(!!!) at 400hz and 9dB at 1khz (midrange dome is stronk). This is a lot -- 6dB is the margin between the 8351B and 8361A for example.
So yes, the KH420 does play much louder at all SPLs than the KH150 even while also having more extension.
P.S. These two speakers are actually a good example of "be careful with distortion measurements" because the KH150 has almost no difference between its 10% and 3% THD capabilities, but the KH420 has quite a bit. So not only are they different capabilities, but the behaviour of their woofers as they near limits seem to be different as well.