More info would be required in order to determine if the overall scoring impact of being an AU on that account is positive or negative.
Clearly, being at such a high % util on a relatively high credit limit account is negative, and % util of revolving credit is weighted significantly in scoring.
Not stated is whether the higher-weighted category of payment history or the lower-weighted category of length of credit are helping or hurting.
Being on time in payments over the last four years does not paint the entire payment history picture. Were their reported derogs four or more years ago that are still affecting scoring? Are there any major derogs on the account?
Is the age of the AU account longer or shorter than your current overall average age of accounts?
Finally, as an additional consideration, any time you have an AU included in your scoring, it automatically results in the credit score no longer being representative of an evaluation of only your own personal credit history. A potential creditor, in reviewing an application for credit, has no way of backing out the impact of the AU on your score, and produce a "real" score that represents an evaluation of only your own credit history. They have the same uncertainty as to its overall effect as you currently have.
Some creditors, such as a mortgage lendor, may even require removal of any AUs as a condition of their underwriting process in order to obtain a score that is based only on your own personal evaluation.
If at all in doubt and you expect to be applying for new credit where the creditor is likely to do a manual review, and thus be aware that your score is based in part on the credit history of another, then ditching the AU account might be advisable, regardless of whether it is increasing your three digit score.
