FWIW, particularly if it was made available as an "option" a bit like you can switch between the PDF engines currently, I don't care if it takes 10 minutes

. In comparison to the *hours* it takes to manually edit calibre converted PDFs currently to fix up the paragraph splitting issues with the existing engine you are going to be way ahead of the curve.
If/when the new C++ engine is finished and provides quality at blinding speed then that could be even better of course. However I think there would be plenty of users who would be willing to sacrifice some conversion speed if the quality stacks up to the advertised goals. Well I certainly would anyway

.