The "best" settings will vary from one drive to the next. Most 1693's seem to be doing well with all settings set to max, which is 16x for all and 12x for DVD-R9.
The consequences of setting all speeds to max are that it does create more wear and tear on the drive, but not so much that you shouldn't see a year or so of good use for ripping a disc or 2 every day. That too will vary by drive.
If a drive is not as good of a reader as others, setting the read speed too high will actually result in longer rip times, because the drive will slow down too often. In that case, setting it to 12x or even 8x can sometimes result in quicker rips because the average speed will be higher.
Try it and see how it goes.