www.makemkv.com

I'm able to use 0=8 to limit the drive speed to 2.0x as I have some discs that are making a grinding noise and failing if I go faster.

What number should I use for 3.0x and 4.0x?

I followed the guide here: viewtopic.php?t=22689

I don't know any more than what's posted in the Drive Speed Control page, but what happens if you set it to 12 or 16?

The number after the "0" represents the MB/s value. 4x speed is about 16MB/s transfer speed. So "8" is about 2x speed.
As far as I observed values lower than "8" seem to have no effect. Speed remains at 2x.