It is the same with CD-R, I guess ... every possible combination of write device/media will have a "sweet spot" speed setting which leads to the lowest possible PI-Sum 8 error rates, signal asymmetry and pit/land jitter (just to mention a few parameters).

To find this "sweet spot" is the challenge ... as an example, when I use TY 4x +R (YUDEN000T01 by Maxell) on my BenQ 1620, they do write better at 8x P-CAV than at 4x CLV. On the opposite, my Plextor 712A will write them better at "pure" CLV speed (4x, 6x).

Anyway, I don't think that 2,4x is the way to go to achieve better overall quality ... if you have to decrease write speed below the media's certificate to get the discs to play, you should definitely change media.