Not sure about the speed thing (except that Ritek G05 is 8x media...the rest are 16x, though).
With the CMC MAG AM3, it may be that it's not fully supported by the firmware, I've heard that MID giving problems before.
Do you have the latest DL13 firmware?
I have a similar problem with my 4167, in that I can't burn at 16x, either - my max is 12x. But I put that down to using a 40 wire IDE cable, rather than the usual 80 wire.
So, check your firmware, and post what cable you're using (40 or 80 wire).