Standard VHS deck + external TBC & comb filter?
 

The VCR buying guide lists some S-VHS decks without TBC, but aside from the built-in comb filter (I already have a good external line TBC and comb filter), is the picture quality and EP/SLP tracking on these any better for digital capture of standard VHS tapes than a good standard VHS deck like the Panasonic AG-2560 or some of the better Sharp models?

I don't think it has anything to do with S-VHS capability or line TBC but more of the quality of the tracking mechanism and its circuitry, I have few long play tapes that did not do well on a Sony normal VCR but my JVC HR-S7600 played them fine, They were odd ball speed to the point where the VCR could not display any of the speed logos on the front display such as SP, LP, SLP, EP ...etc because it was non standard speed, It's so non standard that when I hit pause the frame is a complete static even the dynamic drum system could not compensate for it (out of the DD angle range), that's how non standard it was. Yet the JVC VCR played them just fine.

In theory S-VIDEO (found on S-VHS machines) should produce better captures. This is because the video is stored on tape as separate B&W and color information, even with standard VHS recordings. The S-VHS (with S-VIDEO connection) keeps the signal separate for output (thus no need for comb filter in the capture device). Standard VHS VCR connection (via composite) combines the B&W and color signals read from tape for output only to be separated again by the TBC and/or capture device.

I say in theory because ultimately it depends on the quality and condition of the VCR and the other gear in the capture chain.

Other issues can come into play with EP/SLP and LP recordings including the width of heads used for reading the slow speed recordings and the extent to which the VCR design was optimized for EP/SLP and LP performance. And of course the VCRs auto-tracking.

Sorry in my previous reply I was addressing only the tracking part of the OP's question, I didn't notice that he was asking about the picture quality out of the S-Video port, So yes a S-VHS deck is better in playing back normal VHS tapes than a regular VCR with a composite output mainly because of what was said in the above reply.

That's a good point. Then to get picture quality equivalent to playing that standard VHS tape in an S-VHS deck, a person would have to tap into the signal before the separate B&W and color information gets stuffed into the one composite signal.

Essentially, but it works only if the electronics (amplifiers, equalization, bandwidth, noise level, etc.) up to the point of combination into a composite signal are equivalent, which may or may not be the case. At that point one would have to provide appropriate output buffering circuits to ensure the right impedance, etc.

IMO generally not worth the effort except as a labor of love to a serious hobbiest with the needed electronics skills.