That's strange. I have "Best Speed" as well as slower speeds for Toast 7.
However, it depends on the drive and disks. For my Maxell branded Ritek G05 disks in my Pioneer 110 drive it gives me 4X, 6X, 8X. For the same disks in my Pioneer K04 drive it gives me 4X and 8X only.
What is your drive? Perhaps "Best" is the slowest it's spec'd in the firmware already anyway? eg. 4X maybe? My drives will not give me a 2X option for this media.
P.S. Ritek G05 isn't the greatest media. I burn my Maxell branded Ritek G05 at 4X, regardless of the batch and regardless of the drive. The media simply is not reliable at 8X.