I was searching for some feedback on the same question, so I thought I would respond to you and point you to a good review on the TC 20eIII that I found.
Similar to you I have the D300s body and I use the 70-200mm VR2 lens extensively for sports photography. Though I would love to buy the 300mm f/2.8 VRII lens, as a general enthusiast only its difficult to justify the cost.
Pretty sure I am going to go with the TC-20e III teleconverter.
Here is a link to the review that I found:
Also, a really informative site for Nikon products:
Good Luck and post with your thoughts as I would be curious about what you decide.