I believe that is an accurate analysis, and it shows that the penetrator has exactly the same energy whether the sabot disengages or not. On impact, the penetrator will act as though the sabot were not present (which was my point). Even if mistakenly cemented to the penetrator by manufacturing error, the sabot is of such light and flimsy material that it will simply be stripped off at negligible energy cost.
In fact, bullets have been made as more or less traditional hollow points with a long penetrator embedded (the tail end hanging out the back of the lead slug). The lead slug deforms on impact and the penetrator is released to punch through walls or even armor. I believe they've been shown to penetrate ESAPI plates that stop standard AP rounds of the same calibre.