Welcome to the forum.
When you enable overspeed, it automatically enables SolidBurn for known media. As SolidBurn performs additional checks during the burn at regular intervals, the burning process takes a little longer than when SB is OFF. SB OFF for known media is the default setting, so your previous burns with known 8X media were faster, that’s normal. But…
…But you say it took MUCH longer. So I wonder, are you sure that the burn of your 2.4X media was actually performed @8X? You have to know that Overspeed doesn’t always allow the speed you selected, as SolidBurn first checks that the speed selected is OK for this media (specific tests), and switches to a lower burning speed if it detects that it’s necessary. So maybe your burn took place @4X, or even 2.4X (though I doubt it). Not all burning softwares show the actual burning speed. For example Nero 7 and ImgBurn do show the actual burning speed during the burn, but Nero 6 doesn’t.
Anyway, how can I verify that my burn was successful without calculating MD5 sums for all the files on the HD, then on the disc?
Use the “verify” option in your burning software. It’s not 100% foolproof though.
As for the actual burning quality of your 2.4X media burnt at higher speeds, it’s another story…
Hope this helps