Most if not all problems at high speed seem to be mainly media related. At the risk of repeating myself once more, here’s what I base my conclusions on.
I went through two spindles of the same media and tried all firmwares avaliable for the 40125S, official and overclocked. It seems the burner is most reliable using the original firmware (Z-CLV), but the actual differences are small in CD-Speed Scandisk. As for all the later P-CAV, they all give sensibly similar results.
In all cases, the media is the main limiting factor.
As for different apps, no major differences at a given burn speed, Except when using Norton Ghost (it’s a DOS app and it doesn’t let you set the burn speed). Seems the burns are much slower with all the later firmwares and that at least for this app, the early Z-CLV firmwares work best.