Ok, so I googled and came across this article using one of the old Vivitar 3x TC's.
I have to say, I am rather impressed with the results.
Most of the reviews I read were negative for the 3x, but not the 2x for the reasons stated by posters above.
I realize this person tested with a d***. But, I like to tinker about a bit with my gadgets too so, needless to say, I will play with my old beat-up 3x TC over the holidays.