No. Save your pennies.
I have a tamron b-bar 2x that i have used with a 80-200 hs apo. The af is slow and hunts, the images are ok if you don't crop, but i get a way better quality image if i crop the 80-200 to the same size or bigger, + fast af, + narrower dof.
By the way, how is the image quality of the 300 2.8 with the 1.4x compared to without 1.4x? Is there any slow down of af speed/accuracy, and can you crop the 300 +1.4x much? cheers.