Please note that the connectX-2 adapter is reached its end of life and end of support period following that it was not certified against the latest kernel versions.
Understood Viki. That may have had something to do with why I opened this topic in the Hobby and Home Users Group, where it's common to use gear sourced from (eg) Ebay and similar.
Regarding the problem itself, it's been confirmed by RH staff due to someone else reporting the same problem (I guess with a newer card).
A working solution is in the CentOS bug report too, in case that's of use to people. It also works with the older ConnectX (series 1, not 2) cards, as I also tested with that.