In my Canon times, I regularly used EF 300/2.8L IS USM with Canon EF 1.4x II and EF 2x II teleconverters. When I switched to Nikon and bought AF-S 300/2.8 VR lens together with Nikon's TC20EII 2x teleconverter, I started to think other alternatives to Nikon's teleconverter, because I didn't like the image quality what TC20EII did offer. So I bought Kenko PRO 300 DG 2x teleconverter, it would also fit my Sigma EX 150/2.8 APO DG macro lens. Nikon TC20EII doesn't fit to Sigma 150mm macro, even if you remove the AF-S tap from the teleconverter's mount.
Test procedure principles:
Target has to be big enough, that camera can be far enough, that depth of field does not become a problem. No focusing between taking test shots... Only one liveview checked focusing after changing teleconverter. Shutter release with mirror lockup and remote cord to minimize vibration. Image files directly from memory card to Lightroom, only cropping from center and border, no sharpening etc... then export to JPG-file. Result would be viewed with 100% crops...
AF-S 300/2.8 VR Nikkor
Nikon TC20EII 2x teleconverter
Kenko PRO 300 DG 2x teleconverter
Gitzo gt3540LS + RRS BH-55 PRO
Three 20€ bills taped on the wall