What is an acceptable level for a high end L lens? What are Canon's official tolerances?
I've never heard anything 'official'. But personally, I'd prefer to stay out if double-digit values.
What I can say is that the AF system spec is 'within one depth of focus' for standard-precision AF points, and 'within 1/3 the depth of focus' for the high-precision f/2.8 point(s). Also, each unit of AFMA is 1/8 the depth of focus. So, anything over |8| units of AFMA means the lens is outside the accuracy spec for the AF system.
On my 1D X, the tele end of my 16-35L II needs +7, all other AFMA values are |4| or less, many are zero. On my 7D, they're all |4| or less. On my 5DII, values were also low - but at one point, I dropped the body to the pavement
after which the camera functioned fine, but all my AFMA values were negative shifted by ~10 units, so at that point, I had a couple of -9's and a -11.