Not the most precise of tests:
Simply stayed in the same spot, handheld the K-3II, swapped between lenses while keeping the settings the same. ISO800 / Shutter at 1000 / Aperture at 7.1. I photographed a birdhouse in the corner of my backyard.
DA300 w/ TC
DFA150-450
Here are some parts of the birdhouse zoomed in (DA300 w/ TC on left, DFA150-450 on right):
Focusing speed, from what I could tell was negligible. Without the TC, the DA300 was very sharp (as to be expected).
I am sure I could have done this test better, but for my needs, I am sticking with the DFA150-450. Also take my simple comparison with a grain of salt . There are tons of great images with this combination. My conclusion is that the DFA150-450 focuses more consistently than the DA300 w/ TC, thus resulting in more predictable results.