As one of a seemingly few on net54 who enjoys and seeks out the 'right' cards with qualifiers, let me offer a couple of quick examples as to why the the two-point drop in value doesn't often compute.
According to that theory, a PSA 8 OC and a PSA 6 should be the same price, so I did a quick search on PSA's 'Auction Prices Realized' site to find cards that had similar centering, and grabbed these four examples (not my cards)...
This first pair was graded long ago, and although the centering is decently alike overall, the PSA 8 OC does have much better centering, and look how much sharper the corners are. For a moment, imagine the centering was (literally) exactly the same on both. The card with the qualifier is so much stronger in every other aspect, so it should never sell for the same price as the PSA 6. It is much more 'valuable.'
1969clementeOCcomp02.jpg
In this example, the PSA 6 is a recent grade, so it must be better than the 6 in the previous example, but again, the card with the qualifier is much better in every aspect. The two cards should never command the same price.
1969clementeOCcomp01.jpg
These are non-cherry-picked examples, so you can find a million other cards to either agree with or refute what I am saying, but the basic fact remains:
“All OC cards are equal, but some OC cards are more equal than others.” 