"80-wire cables" issue when using SATA drives in PATA Thinkpads

[ Not sure where to send this question.
This mailing-list seems to be much more oriented towards development of libata code than its use, but the question seems too technical for other foras. ]

I'm trying to use a SATA M.2 drive in a Thinkpad X30 (via a PATA->M.2 enclosure) and am bumping into a ~30MB/s bandwidth limit that's apparently imposed by the BIOS for "lack" of an 80-wire cable.

This is mentioned at the end of
https://www.thinkwiki.org/wiki/How_to_put_SATA_in_old_ThinkPads
and links to
https://forum.thinkpads.com//viewtopic.php?f=31&t=115478&p=741729

which mentions this as a known problem.
Is there some known workaround for it?

I tried to boot with libata.force=80c and it does result in hdparm claiming that udma5 is used instead of udma2, but that seems to be a lie because the actual bandwidth I see is still ~30MB/s.

How can I diagnose the problem further (and ideally fix it)?

Stefan