It is 80-400mm VR. Not 70-400mm

I have also tested the new TC-20E III.  Sold my TC-17EII the next day. 

Personally, I don't really like these with -1 or more EV.  They are too dark.  I understand you use that trick to get the speed up.  Did you shoot in raw?  May be you could add more light digitally using a software.