Get CD Bremse and force the drive to read at 4x, I'll bet the results are very close then.
I have to slow my 166 to 4x and lock the speed then it gets fairly consistent and low error rates with good disks. My 811 still gets lower error rates on plus media but I think the 166 drive and probably 167 can give good ideas of whether the disk is good and will work in most players/readers.
I also like to do a transfer rate graph with them to see if the disk really can be read well at full speed. Many disks that test extremely well still don't give a smooth graph when read by my 166, however they usually play fine and I get good graphs from my other slower readers. My 166 may be a bit funky so who knows, it is 6 months old and I use it a lot.