I'm still not totally clear on how one proves that a full frame TBC is working well other than "not adding noise", "not dropping frames", and possibly visibly noticeable horizontal jitter improvement. But if you aren't dropping frames and can't see anything visually off with the image, what benefit is there to the the TBC?
Thing is, at least in my setups, not using a TBC at all usually doesn't result in any dropped frames. In fact, I almost always see "inserted frames" whether using a TBC or not which I believe is due to some sort of clock difference on the PC/Virtual dub where it thinks the frame rate is coming in slightly slow, so it regularly will insert a frame every 90-120 seconds and that ticks up by one every few minutes.
I guess the other thing you have to ask yourself is if the incoming frame rate is irregular/slow and the TBC ends up doing frame inserting to keep output it stable, is that any different from having VirtualDub
do the frame insertions instead?
You'd think the advantage of the full frame TBC would be for when frames are coming in "too quickly" that it can buffer them down to the correct rate without losing/dropping them whereas the capture card direct without a TBC would have to drop them because it is unable to buffer frames that arrive too quickly. However, if you are't seeing any dropped frames in your capture setup to begin with without the TBC, doesn't that indicate that the TBC isn't needed?