I decided to go ahead and get the AVT-8710 TBC, and to my surprise it made no improvement at all. This surprised me seeing how much better the picture was when recording to the svideo-input on my JVC camcorder. I assumed the JVC was just a cheap line TBC - but it seems far better than the AVT-8710 full frame TBC.
The link below contains a montage with the three ways I captured
1) (left pane) Sony EV-S900 Hi8 ->
Pinnacle 500-USB DV capture
2) (middle pane) Sony EV-S900 Hi8 -> AVT-8710 TBC ->
Pinnacle 500-USB DV capture
3) (right pane) Sony EV-S900 Hi8 Svideo -> JVC Camcorder mpeg2 capture
https://docs.google.com/leaf?id=0B9X...MTE4N2Y0&hl=en