D7000 poor focus with 100-300 lens

I just recently got a used Sigma 100-300 f/4 lens for my D7K. Working near an Air Force Base allows me photo opportunities like these. I am not too pleased with the focus achieved here. This was taken at dusk, so not a ton of light but certainly should be enough for the PDAF to lock on. I was not using my tripod (handheld), but I figured 1/800 should be fast enough to cancel out any camera shake from handholding. I would really like to take this shot around 1/100 so the propellers have some movement instead of being frozen. Please help me figure out what I'm doing wrong. Here's the critical settings:

300mm
1/800s
f/4
ISO 500
AF-S single point (center sensor on left wing of aircraft)

Original JPG

100% crop